The impact of malnutrition on morbidity and mortality associated with acute respiratory tract infection (ARI) was studied in Filipino children less than 5 years old. Malnutrition measured by weight-for-age Z-scores of less than -3 SD and less than -2 SD from the National Center for Health Statistics median reference population was associated with the following significant relative risks of morbidity: 1.24 (95% confidence interval [CI] = 1.14, 1.34) and 1.14 (95% CI = 1.08, 1.19), respectively, for ARI; and 1.9 (95% CI = 1.46, 2.39) and 1.2 (95% CI = 1.03, 1.47), respectively, for acute lower respiratory tract infection (ALRI). These risk ratios remained significant when adjusted for age, crowding, and parental smoking. Malnourished children with severe ALRI also had a mortality risk two to three times higher than that of healthy children. This risk remained significant even when adjusted for significant predictors of mortality, including clinical complications, concurrent measles, severe infections, and female gender; and for clinical factors, including extent of pneumonic infiltrates, dehydration, and hepatic enlargement. These findings underscore the importance of nutritional intervention in the control of morbidity and mortality among patients with ARI.