The synergism between nutrition status and hospital admissions due to diarrhea and pneumonia was studied in a population-based birth cohort of greater than 5000 children in southern Brazil. Children were identified soon after birth in 1982, and data on nutrition status (weight and length) and hospital admissions were collected in 1984 and in 1986. Diarrhea admissions were stronger predictors of malnutrition than were pneumonia admissions, but malnutrition was a more important risk factor for pneumonia than for diarrhea. All associations were stronger in the first 2 y of life, although the early effect of severe diarrhea and pneumonia on nutrition status could still be detected in the fourth year of life.