To assess the effects of common infections on dietary intake, 131 Peruvian infants were observed longitudinally. Home surveillance for illness symptoms was completed thrice weekly, and food and breast-milk consumption was measured during 1615 full-day observations. Mean (+/- SD) energy intakes on symptom-free days were 557 +/- 128 kcal/d (92.4 +/- 26.5 kcal.kg-1.d-1) for infants aged less than 181 d and 638 +/- 193 kcal/d (77.7 +/- 25.7 kcal.kg-1.d-1) for infants aged greater than 180 d. Statistical models controlling for infant age, season of the year, and individual showed significant 5-6% decreases in total energy intake during diarrhea or fever. There were no changes with illness in the frequency of breast-feeding, total suckling time, or amount of breast-milk energy consumed. By contrast, energy intake from non-breast-milk sources decreased by 20-30% during diarrhea and fever, and the small decrements in total energy consumption during illness were explained entirely by reduced consumption of non-breast-milk foods.