Abstract: The incidence of death of broiler birds above 40 days suddenly increased in the month of July-October 2009 in Kathmandu valley. Birds that were presented for post-mortem examination at the Central Veterinary Laboratory, Tripureswor, Kathmandu were usually found dead on their backs with wings out-stretched. Gross abnormalities recorded on post mortem examinations were: muscle oedema, pulmonary, renal and liver congestion, dark black to pale yellow streaked liver bile filled gall bladder and congestive splenomegaly, blood clot in atrium haemorrhage in duodenal muscle, whitish yellow pasty fluid in proventriculus gland, greenish coloration marked intact feed particles in gizzard and excessive mucous filled swollen intestine. Incidence rate was recorded between 1.5-2.5% of the flock. The mean mortality rate due to sudden death syndrome was 1.3-9.6%. Penicillium sp. and Aspergillus sp. with CFU ranging from 56x10^-4 to 62x10⁵, to uncountable mold count, E. coli, Streptococcus sp. and Staphylococcus sp. were the usual organisms isolated from culture samples of liver, lung, spleen and proventriculus.