Ureaplasma urealyticum organisms (ureaplasmas) and Mycoplasma hominis organisms (mycoplasmas) were sought in mid-stream urines collected from 200 men and 200 women attending hospital with conditions of a non-venereal nature. In addition, the urines from 100 male and 100 female healthy volunteers were examined. Overall, ureaplasmas were isolated four times more often than mycoplasmas. In individuals less than 50 years of age, the organisms were found in about 20% of men and about 40% of women. In individuals 50 years or older, they were found about one-third to one-half as frequently. Centrifugation of urine and examination of the resuspended deposit did not increase the isolation rates. In men, the numbers of organisms in the urine were usually small (less than or equal to 10(3) c.c.u./ml) with less than tenfold more in the urine of women. The occurrence of 51- greater than 1000 leucocytes per mm3 in some of the urines was not associated with either the presence or an increased number of ureaplasmas/mycoplasmas, whereas they were associated with the presence of 10(5) or more bacteria/ml. The significance of these findings in the context of defining the role of ureaplasmas/mycoplasmas in genital-tract disease is discussed.