Acute hemorrhagic cystitis (AHC) characterized by gross hematuria and symptoms of bladder inflammation occurs in children as a self-limited disease that must be differentiated from serious renal disorders. Of 69 children with AHC, adenovirus type 11 was recovered from the urine of ten, and adenovirus type 21 from the urine of two. Two of 42 control children also had adenovirus type 11 viruria. An increase in adenovirus type 11 neutralizing antibody was detected in four of four AHC patients with adenovirus 11 viruria, in one of five children with bacteriuria, and in one of 19 children from whom no agent was recovered. Twelve patients had Escherichia coli bacteriuria. Bacteria were not isolated from the urine of 18 control patients. Overall, adenovirus or E coli infections were detected in 39% of AHC cases.