A hospital-based inpatient and outpatient study of 1,492 cases of acute lower respiratory tract infection (ALRI) was conducted from November 1986 to March 1988 in two hospitals in Rawalpindi and Islamabad, Pakistan. Specimens of nasopharyngeal aspirate were processed for viral studies in all cases; blood cultures were performed in 1,331 cases; and urine was obtained for detection of bacterial antigen in 378 cases, but 227 of these samples had bacterial contamination and were discarded. Respiratory syncytial virus was identified in 33% of cases, and Haemophilus influenzae and Streptococcus pneumoniae were identified in 9.6% and 9.9% of cases, respectively. Nonencapsulated H. influenzae accounted for 32% of the Haemophilus isolates, and type b was the only encapsulated H. influenzae strain identified. Of the S. pneumoniae serotypes isolated, 31% are not included in the currently available polyvalent pneumococcal vaccine. No clinical characteristic was demonstrated to be a reliable indicator for bacterial ALRI.