Cryptosporidium parvum causes persistent diarrhea in young children in developing countries. To determine the interaction between nutritional status and cryptosporidiosis, an open cohort of 1064 children younger than 3 y of age was followed for 1441 child-years by weekly diarrhea recall visits. A total of 5072 weight and 4264 height measurements was made. There were no tendencies of low weight (P = 0.38) or height (P = 0.16) in children who acquired cryptosporidiosis. Cryptosporidiosis in infancy was accompanied by an estimated weight loss of 392 g (95% CI: 247, 538 g) in boys and 294 g (95% CI: 109, 479 g) in girls, corresponding to 3.7% and 2.9% of mean weight, respectively, at 2 y of age. No significant catch-up growth covered for this loss in weight. A similar effect in linear growth was shown (P = 0.02). Although it has been suggested that the effect of infections on nutrition is usually transient because of catch-up growth, the present study suggests that cryptosporidiosis in infancy has a permanent effect on growth.