Of 52 clinically healthy, ambulant elderly subjects with urine that was sterile or contained 2.4 × 105/min. Subjects with bacteriuria had decreased blood pressure, rate of creatinine clearance, maximal level of urinary creatinine, and maximal urinary flow. Twenty-nine of the nonbacteriuric and eight of the subjects who developed bacteriuria after diuresis were retested one to two years later. No subject had received antibacterial agents in the interim. The infections of four of the original subjects who had developed bacteriuria had cleared up and showed functional improvement. The remaining four subjects who had developed bacteriuria during the first study and three subjects who had been nonbacteriuric during the first study had concentrations of bacteria ranging from 0.05 × 105 to 1.80 × 105/ml before diuresis in the second study, and excreted increasing numbers of bacteria (from 0.12 × 105 to 9.0 × 105/min) in response to the diuretic loading. Two of the subjects who were bacteriuric in both surveys were infected with different bacteria during the two studies. The data show the high frequency of occult bacteriuria in ostensibly healthy elderly patients that is revealed only by forced diuresis, the detectable functional deficits, the improvement after clearing of the infections, and the rapid turnover between infected and clear periods.