Midstream samples of urine from 185 acutely dysuric women and 89 symptom-free controls were screened according to the modified criteria of Kass by both conventional and microaerophilic culture. Among the 185 symptomatic women, coliform bacilli were isolated from 125 (67.5%) and in 45 (36%) of the latter the concentration of these organisms in the urine was less than 10(8)/l. Fastidious organisms were isolated in pure and mixed cultures from 25 (13.4%) of the 185 patients and from 4 (4.5%) of 89 controls. Staphylococcus species were isolated from 10.8% (9.7% S. saprophyticus; 1.1% S. epidermidis) and Ureaplasma urealyticum from 2.7%. All patients infected with conventional organisms but only 30.4% of those yielding fastidious organisms had pyuria. The isolation rate of fastidious organisms tended to be higher (P = 0.055) in symptomatic women that in symptom-free controls.