A 17-month (from January 2002 to May 2003) longitudinal study was undertaken to elucidate the epidemiology of important endemic and epidemic diseases affecting semi-scavenging chickens reared in the Participatory Livestock Development Project area in Bangladesh. This project was implemented in 17 northern and north-central districts of Bangladesh, under which 361 839 rural poor people were assisted to undertake poultry-rearing activity as a tool of poverty reduction. Of the total beneficiaries 93% were "key rearers". A key rearer is defined as a beneficiary who generally rears 10 to 13 hens in a semi-scavenging system with little additional feed supply. Households of 650 key rearers and some chick rearers were observed. During the study period 1227 birds, which belonged to different age, breed and sex categories, were found dead as a result of disease occurrence. From every dead bird organ samples such as the liver, heart, spleen, brain lung, trachea and bursa of Fabricius were collected. The incidence rate of mortality was 0.01976 per bird-months at risk. Of the total deaths 58.44% had single or mixed type of infections. Newcastle disease had the highest proportional mortality rate (15.81%). The proportional mortality caused by fowlpox, fowl cholera, salmonellosis, colibacillosis, aspergillosis, infectious bursal disease, mixed infections and undiagnosed cases were 8.96%, 6.76%, 7.09%, 6.93%, 0.33%, 2.04%, 10.51% and 41.56%, respectively. Newcastle disease affected a significant higher proportion (18.81%) of birds older than 60 days of age (P=0.00). Younger birds (age < or = 60 days) had a higher proportional mortality due to fowlpox and infectious bursal disease than older birds (P=0.00). Sonali (male Rhode Island Red x female Fayoumi) birds reared under the semi-scavenging system had a higher infection rate with Newcastle disease virus compared with indigenous and Fayoumi birds (P=0.00). Fowlpox was more prevalent in Fayoumi birds compared with Sonali. Surprisingly, Newcastle disease was more common in the vaccinated birds rather than the unvaccinated birds.