The relationship between childhood illnesses and growth increments in length and weight was investigated in a 13-month birth cohort of rural Mexican children. Increments in length and weight for each year from birth to three years were related to high and low frequencies of reported time ill during the same period. Seventy-two of the 276 children had already been characterized as exhibiting "growth failure" relative to other members of the cohorts, and this was considered as a separate factor in the study. We found that upper and lower respiratory infection did not affect incremental gain in height or weight. A high frequency of diarrheal infection was found to reduce weight gain, although gain in height was not affected. Relative to the total sample, the average child with a high frequency of diarrhea achieved only 95 per cent of expected body weight age three; a chidl with both growth failure and high diarrheal frequency reached only 90 per cent of expected body weight at age three.