This epidemiological study has been carried out in urban and rural areas of West Tripura district, to determine the incidence, causes, risk factors, morbidity and mortality associated with acute respiratory infection (ARI) and impact of simple case management in children under 5 years of age. The annual attack rate (episode) per child was more in urban area than in rural area. Monthly incidence of ARI was 23% in urban area, 17.65% in rural area. The overall incidence of ARI was 20%. The incidence of pneumonia was 16 per 1000 children in urban area and 5 per 1000 in rural area. The incidence of pneumonia was found to be the highest in infant group; 3% of ARI cases in rural area and 7% in urban area developed pneumonia. Malnourishment in urban area was 54% and in rural area 65%. Malnourished children have higher likelihood for developing respiratory infection. The relative risk (RR) of developing pneumonia was 2.3 in malnourished children. Most children (59%) had been immunised with measles and diphtheria, pertussis and tetanus (DPT) vaccine earlier. The immunisation had a protective role in pneumonia. The RR was 2.7 in non-immunised group. Air pollution of the urban area had stronger relation for bronchial asthma than pneumonia. Breastfeeding had protective role in pneumonia and severe disease. Bottlefeeding had greater risk of developing pneumonia. Lower socio-economic status had the greater risk of ARI episodes. ARI was decreased as the per capita income increased. An increase in magnitude of ARI was observed with the decrease of literacy rate. Administration of co-trimoxazole for pneumonia case by trained health worker using simple case management strategies can reduce deaths from pneumonia significantly. Health education can change health care seeking behaviours and attitude of parents and other family members to take care of the ARI child in the home itself for preventing pneumonia death.