The present prospective study was conducted at two urban slums of Delhi, Kusumpur Pahari and Kathputly Colony, in the peak winter season from November 1994 through February 1995. We studied 642 infants to determine the incidence of acute lower respiratory infection (ALRI) and its relationship to indoor air pollution due to fuel used for cooking (wood or kerosene). In Kusumpur Pahari, there were 317 children (142 wood and 175 kerosene), including 64 controls and 78 cases of ALRI in the wood fuel group and 81 controls and 94 ALRI cases in the kerosene group (p > 0.05). Out of 316 children in Kathputly Colony (174 wood and 142 kerosene), there were 33 and 45 ALRI cases in the wood and kerosene groups, respectively (p < 0.05). Controls were children without ALRI and were used as controls in different groups. The demographic data and risk factors, namely, nutritional and immunization status, were comparable in ALRI cases and controls in both study areas. Pneumonia was the most common ailment in all the groups. Bronchiolitis was reported in 22.5% of the wood group and 27.1% of the kerosene group in Kathputly Colony versus 13.7% in the wood group and 12.1% in the kerosene group in Kusumpur Colony. Only one case of croup was reported from Kusumpur Pahari among wood users. The duration of illness was longer in the Kusumpur Pahari due to poor compliance, feeding, and child rearing habits. In conclusion, a higher incidence of ALRI was reported in kerosene users in Kathputly Colony, a high pollution area; however, the reasons for the differences observed need further elucidation.