In three studies, 616 patients (610 female and six male) presented with symptoms suggesting urinary tract infection (UTI). Of these, 321 had greater than 100 X 10(6) bacteria per litre in a midstream urine sample. None of the presenting symptoms could be considered to be a reliable indicator of bacteriuria. Ninety percent of infected urines and 44 percent of uninfected urines had a raised white cell concentration. Escherichia coli was the most common infecting organism (71.3 percent). Next were Gram positive cocci at 15.6 percent, Proteus mirabilis (7.5 percent), and Klebsiella-Enterobacter species (0.7 percent). At follow-up over 30 days 25.6 percent of patients initially infected again had infected urine. The organisms isolated from initial MSUs were sensitive to most antibacterial agents by disc-sensitivity testing. The expected urine levels of drugs used in the study greatly exceeded the measured minimum inhibitory concentration.