A prospective study of the pathogenic role of anaerobic bacteria was undertaken by suprapubic bladder aspiration in 100 patients prior to routine endoscopic urethral instrumentation. Anaerobic and/or micro-aerophilic bacteria were isolated from the suprapubic specimens of 11 patients (greater than 10(5) organisms/ml from 8), with identical species growing heavily from corresponding cystoscopic specimens and/or urethral swabs. Previous cystoscopies in all 11 patients and the presence of bladder tumour at the time of sampling in six were significantly and independently associated with the isolation of anaerobic bacteria from suprapubic specimens. It was concluded that transurethral instrumentation entails a definite risk of anaerobic urinary infection in addition to the recognised risk of aerobic urinary infection. The findings suggest that the antibiotics used as prophylaxis for or treatment of patients undergoing transurethral procedures should be effective against anaerobic bacteria.