Background: Malnutrition in the early years of child life can cause long-lasting deleterious effects which may prevent behavioural, motor, cognitive development, educational achievement and reproductive health. Children with severe acute malnutrition (SAM), which is associated with delayed growth and development, often have multiple micronutrient deficiencies, including vitamin D deficiency. According to UNICEF and WHO joint malnutrition estimates for 2016 in Pakistan, 10.5% of children are wasted, 45% are stunted and 31.6% are underweight. If untreated, severe under-nutrition can progress to irreversible effects, with delay in development thereby declining upcoming productivity of these children and worsen the economic burden of country. Therefore, it is important to find predictors for malnutrition to properly address this problem. There are insufficient national statistics on the developmental outcome of severe acute malnutrition (SAM) among children in Pakistan as well as randomized control trials of vitamin D supplementation in growth along with development of SAM children are lacking. So we have tried to explore in this study whether supplementation of vitamin D3 (cholecalciferol), in combination with “ready-to-use therapeutic food (RUTF)”, would increase child growth along with developmental status during the rehabilitation phase of SAM. Clinical trials in SAM with supplementation of vitamin D have not carried out in this population before. Methods: This study was designed in to two phases. First phase was cross- sectional with the aim to reveal the impact of malnutrition on development quotient of children and to explore the dietary and socio demographic factors responsible for severe acute malnutrition and developmental quotient of children. In second phase of study we carried out a “randomised, placebo-controlled, trial of vitamin D3 supplementation” in 185 children between 6-59 months of age with uncomplicated severe acute malnutrition, in southern Punjab, Pakistan. Children were randomly allocated to receive either two oral doses of 200,000 IU vitamin D3, or placebo, along with RUTF, at 2 and 4 weeks. Participants and study staff were unacquainted of treatment assignment. The primary outcome was the proportion with weight gain >15% of baseline and the secondary outcome were mean weight-for-height/length z-score and global developmental status. Developmental quotient of children (Assessed with the Denver Development Screening Tool II) were done at start of study and at end of 2 months. Structured sociodemographic and nutritional questionnaire were used to collect information for predictors on same trial population. “This study is registered with ClinicalTrials.gov, number NCT03170479”. Findings: Out of 194 kids initially randomly enrolled in the study, 185 kids completed follow-up and data records of these 185 kids were included in the analysis. So out of 185 children, 69 (37.3%) have normal developmental, 108 (58.4%) had suspected delayed development and 8 (4.3%) had untestable profile in overall developmental score. Random allocation of children were done in vitamin D3 group (n=93) or placebo group (n=92).Vitamin D3 did not influence the proportion of SAM kids gaining >15% weight from baseline (relative risk [RR] 1.04, 95% CI 0.94-1.15, p=0.47) but it did increase weight-forheight/length z-score (adjusted mean difference 1.07, 95% CI 0.49-1.65, p<0.001) and reduce the proportion of participants with delayed global development (adjusted RR [aRR] 0.49, 95% CI 0.31-0.77, p=0.002), delayed gross motor development (aRR 0.29, 95% CI 0.13-0.64, p=0.002), delayed fine motor development (aRR 0.59, 95% CI 0.38-0.91, p=0.018) and delayed language development (aRR 0.57, 95% CI 0.34-0.96, p=0.036). In sociodemographic and nutritional questionnaire results indicate that weight for height is strongly associated with the family income β - 0.16 with {95% CI (-0.89 to -0.04) p=0.03} and weaning practices β -0.21 {95% CI (-1.14 to 0.19) p=0.01}.In length/height for age (stunting) z-score the significant factors are,family monthly income β -0.16 {95%CI (0.26 to 1.08)p=0.04} mother knowledge of complimentary diet β 0.15 {95%CI (0.25 to 0.96) p=0.03} house hold food security β 0.16 {95%CI (0.11 to 1.48) p=0.02} and exclusive breast feeding practices, β -0.22 {95%CI (-1.47 to -0.30) p=0.00}. Conclusion: There was not any significant difference among two groups in the primary outcome, however high-dose vitamin D3 supplementation increased mean weight gain and the developmental status of children receiving standard therapy for uncomplicated SAM in Pakistan. Further researches are required to determine whether positive outcomes can be replicated in other settings. Moreover, developmental screening ought to be vital for primary healthcare system, specifically in high risk malnourished children and policy makers considering for betterment in children nutritional status should promote healthseeking practices and knowledge of families in this regard in Pakistan.