The association between low initial plasma zinc concentration and risk of morbidity over the subsequent 3 mo was examined in a cohort of 116 children aged 12-59 mo recovering from acute diarrhea. Children with low initial plasma zinc (< or = 8.4 micromol/L) had more episodes of diarrhea [risk ratio (RR): 1.47; 95% CI: 1.03, 2.49) and severe diarrhea, defined as passage of > or = 5 liquid stools in a 24-h period, (RR: 1.70; 95% CI: 1.06, 2.72) than did children with normal plasma zinc (> 8.4 micromol/L). The mean prevalence rate of diarrhea associated with fever was 4 times higher in the zinc-deficient group (P = 0.01). Overall, the difference in the number of episodes of acute lower respiratory tract infections (ALRIs) between the two groups was not statistically significant (RR: 1.76; 95% CI: 0.88-3.53) but the mean prevalence rate of ALRIs was 3.5 times higher in children with low plasma zinc (P = 0.05). The increased risk of diarrhea and ALRIs episodes in zinc-deficient children was larger in boys than in girls. These results show that children with low plasma zinc concentrations are at risk for increased diarrheal and respiratory morbidity.