To evaluate the magnitude of copper and zinc losses during acute diarrhea requiring hospitalization, we studied 14 infants, 3 to 14 months of age, and compared them with a control group of 15 infants of similar age, birth weight, and nutritional status. Metabolic balance studies were conducted in the study group during an initial 48 hours (period 1) and on days 6 and 7 after admission (period 2). The control group was studied after recovery from respiratory disease. Copper and zinc content of feces, urine, and food samples was measured by atomic absorption spectrophotometry. Mean (+/- SD) fecal losses were higher for period 1 in the diarrhea group than in control subjects: Cu 55.7 +/- 21.2 versus 28.8 +/- 6.7 micrograms/kg/body weight/day (p less than 0.01); Zn 159.4 +/- 59.9 versus 47.4 +/- 6.4 micrograms/kg/day (p less than 0.0001). For period 2, Zn losses were similar in both groups, but Cu balance remained negative only in the study group. Retention of Zn for the study group went from -21.2 +/- 46.7 in period 1 to 204.5 +/- 103.0 micrograms/kg/day in period 2 (p less than 0.0001), and fecal weight decreased from 70.5 +/- 20.6 in period 1 to 36.8 +/- 20.0 gm/kg/day in period 2. Fecal weight and fecal losses were correlated: r = 0.71 (p less than 0.01) for Cu and r = 0.81 (p less than 0.001) for Zn. Plasma mean Cu and Zn levels were low in period 1 but rose in period 2, especially for Zn. A negative correlation was found between fecal Zn losses and plasma Zn: r = 0.74 (p less than 0.001). We conclude that acute diarrhea leads to Cu and Zn depletion and that plasma levels and Cu balance remain abnormal a week after admission.