Among 352 residents of a home for the elderly, who were fully mobile and devoid of most predisposing factors for bacteriuria, 19% of men and 27% of women had two positive cultures within two months. Positive conversion at one year among men and women negative at entry was 11% and 23% respectively, negative conversion of bacteriuries 22% and 27%. Subjects bacteriuric at entry but sterile at six months had a 77% (men) and 44% (women) reinfection rate at 12 months. The data indicate firstly that a significant proneness to infection does exist in old age, coupled with a lesser trend toward spontaneous cure; the latter is equal to that of younger ages. Thus, prevalence rises steadily in old age. Secondly a previous history of bacteriuria in a subject with currently sterile urine increases his chances of reinfection or recurrence two to seven times compared to those of subjects without past infection.