Cell-mediated immunity, as assessed by delayed cutaneous hypersensitivity, can be diminished by malnutrition and viral infections. In turn, decreased immune functioning might lead to more frequent or more severe infectious diseases. Delayed cutaneous hypersensitivity was assessed in young Peruvian children by simultaneous application of seven standardized antigens and a negative control (Multitest CMI®). Response to tuberculin was frequent and was higher in children vaccinated with Bacillus Calmette-Guerin, response to tetanus or diphtheria toxoids was also good, especially in children who had received at least two doses of diphtheria-tetanus toxoids-pertussis vaccine. Two summary assessments, the number of positive responses and the sum of indurations of all positive responses provided useful measures of delayed cutaneous hypersensitivity. Responsiveness, as assessed by these summary measures, was inversely related to the incidence of diarrhea, identified by household surveillance for the 6 months after the skin test. Undernutrition, as assessed by weight for age or length for age, was also a significant determinant of the incidence of diarrhea, but not the duration of episodes, in this group of study children. Depressed cell-mediated immunity and malnutrition may be important risk factors for diarrhea in developing country children.