A retrospective, hospital-based case-control study was used to investigate whether there were any clinical characteristics that could distinguish Cryptosporidium parvum-infected children with diarrhea from other non-C. parvum-infected children with diarrhea. Ten percent (24 of 248) of children admitted to a rehydration ward at Cayetano Heredia University Hospital, Lima, Peru, were infected with C. parvum. The 24 patients infected with C. parvum (cases) were matched to an equal number of noninfected patients (controls). C. parvum-infected patients were more likely to be malnourished than were children without this infection (P less than 0.05). Also nosocomial infection caused by C. parvum occurred in three severely malnourished patients, two of whom died. No other clinical or laboratory characteristics were found that would distinguish children with diarrhea caused by C. parvum from other children with diarrhea. In children hospitalized for diarrhea C. parvum infection occurs most frequently in malnourished children.