Newcastle disease virus (NDV) and Infectious bronchitis virus (IBV) are major cause of economic losses to poultry industry worldwide. Pakistan is endemic for both viruses, and these are causing a serious alarming threat to the poultry producers. The present study was aimed to understand the severity of pathogenesis caused by these viruses either alone or as a co-infection in non-vaccinated and vaccinated broiler chickens upon experimental inoculation. For this purpose, representing different poultry houses (PH) in and around district Lahore, field prevailing viruses were isolated and characterized. The mean death time (MDT) for NDV and embryo infective dose rate (EID50) for both viruses were found to be 49.2 to 56.8 hrs and 10-5.76 and 10-6.03, respectively. After confirmation through biological and molecular assay (RT-PCR), the viruses were propagated independently in nine day-old embryonated eggs and later processed for the estimation of its pathogenicity. In the first experiment, a total of 210 day-old broiler chicks were reared under experimental shed and were divided into six groups of 35 birds each. The birds were challenged with the viruses on day 21 through nasal and ocular route and were monitored for clinical signs of the disease. Tissue samples were taken from the infected birds after 2nd, 4th and 6th day post inoculation (dpi) for histopathology and the virus detection through immunohistochemistry (IHC). Clinically, respiratory signs were more severe in IBV infected birds whereas both respiratory and nervous signs were evident from birds infected with NDV. However, high severity of infection was observed in those birds infected with both NDV and IBV as compare to those birds infected with individual virus at a time. Histopathological examinations showed variable degree of congestion and haemorrhages in the trachea, lungs, proventriculus caecal tonsils and kidneys. These observations were correlated with the presence of viral antigens in the tissues through immunohistochemistry. In the second experiment, the vaccinated birds (LaSota strain for NDV and Massachusetts strain for IBV) were challenged with NDV and IBV strain. Following infection with individual and co-infection of NDV and IBV, respiratory signs including gasping, sneezing and coughing were observed in all virus-infected chickens. At the histopathological examination, mild to moderate pathogenesis of NDV and IBV infection was observed in the collected tissue samples. The gross clinical symptoms and histopathological lesions even in vaccinates indicated that current vaccine strains are not able to protect birds from challenge and that the currently used vaccine provides partial protection upon challenge from the field strains.