To define the uropathogens of various childhood populations and their antibiotic susceptibility, 646 episodes of urinary tract infections (UTI) were studied. Of the community-acquired UTI 78% were caused by Escherichia coli and 12% by Klebsiella whereas only 65% of hospital-acquired UTI were caused by E. coli (P less than 0.01), and other pathogens, including Pseudomonas, were more common. In children with UTI who did not have an underlying disorder, most infections were caused by E. coli and Klebsiella species. Children with urinary malformations or urinary catheters or those who developed UTI while receiving antibiotic prophylaxis had fewer E. coli infections and more infections caused by other pathogens, including Pseudomonas (P less than 0.01). Children receiving antibiotic prophylaxis had also significantly more Enterococcus and Acinetobacter infections (P less than 0.001), and children with urinary catheters had more Enterobacter infections (P less than 0.05). Isolates of these risk groups showed increased resistance to antibiotics. Only 30-53% were susceptible to trimethoprim-sulfamethoxazole, which is usually recommended for UTI; 19 to 25% and 27 to 66% were susceptible to ampicillin and cephalothin, respectively. In contrast uropathogens of immunocompromised children did not differ significantly from those of children with no underlying disturbances, nor did they show distinct antibiotic susceptibility patterns.