We prospectively examined the incidence of bacteriuria in malnourished patients between 6 months and 5 years of age. For each patient, a normally nourished control matched for age, sex, and presence of fever and diarrhea was included. Of 112 patients (65 boys), 55 had moderate and 57 had severe malnutrition; 43 had diarrhea and 35 had fever. Clean-catch and suprapubic urine specimens were examined microscopically and cultured. Significant bacteriuria was found in 17 (15.2%) malnourished and 2 (1.8%) control subjects ( P<0.01). The incidence of bacteriuria in malnourished and normally nourished subjects with fever was 28.6% and 5.7%, respectively ( P<0.05). The risk of bacteriuria increased significantly with the severity of malnutrition and in patients with diarrhea. Bacteriuria was associated with symptoms (70.6%) and elevated levels of acute-phase reactants (88.2%), indicating the presence of urinary tract infections (UTI) rather than asymptomatic colonization. Our observations show that malnourished children, particularly those with fever, are at risk for UTI. Urinalysis is useful for screening for UTI in these subjects. Urine culture should be performed in patients showing an abnormal urinalysis, and if the likelihood of detecting bacteriuria is high (as in patients with fever or diarrhea). Significant bacteriuria in malnourished subjects should be treated with appropriate antimicrobials.