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Recently, high mortality was recorded in broiler flocks in various areas of Pakistan. The samples from six broiler flocks were studied. The blood samples collected were analyzed for antibodies to Newcastle Disease Virus (NDV), Avian Influenza Virus (AIV), Infectious Bronchitis Virus (IBV), Infectious Bursal Disease Virus (IBDV), mycoplasma gallisepticum (MG), Mycoplasma synoviae (MS) and Salmonella organisms (SPG). It was found that the samples had no antibodies against NDV, AIV, MG, MS and SPG but variable levels of antibodies were recorded against IBV and IBDV. Bacteriological examination of the respiratory organs of clinically sick birds yielded Haemophilus and pathogenic E. coli. The absence of any pathogen activity in filtrates of 0.2 and 0.1 µm inoculated through CAM and CAS routes in embryonated eggs ruled out the possibility of the involvement of AIV and NDV. Unfiltered homogenate and 0.45 µm filtrate activity indicated the presence of Mycoplasma in the homogenate. It is concluded that: I. The problem primarily resulted from the interplay of Mycoplasma, IBDV, IBHV and IBV, 2. Quality of the chicks in carrying vertical Mycoplasma infection played basic role in the development of the problem, 3. The associated bacterial pathogens i.e. Infectious Coryza and Colibacillosis played precipitating role in the problem, 4. Extreme environmental temperature played a conducive role in the episode and 5. Predisposing role of mycotoxins in the malady cannot be overlooked.