Bacteriuria is common in chronically catheterized patients and is associated with both acute and chronic complications. Of 605 consecutive weekly urine specimens from 20 chronically catheterized patients, 98% contained bacteria at high concentrations and 77% were polymicrobial. The mean interval between new episodes of bacteriuria was 1.8 weeks. Most species of bacteria caused five to seven new episodes of bacteriuria per 100 weeks of catheterization. Even though access to the catheter lumen was similar, the duration of bacteriuric episodes varied greatly by species. Of the episodes of bacteriuria caused by nonenterococcal gram-positive cocci, greater than 75% lasted less than one week. Mean durations of episodes of bacteriuria due to Escherichia coli, Proteus mirabilis, and Pseudomonas aeruginosa were four to six weeks, whereas those due to Providencia stuartii averaged 10 weeks and ranged up to 36 weeks. Thus, the very high prevalence of bacteriuria--virtually 100%--was a result of a high incidence caused by many different species combined with the prolonged residence of some gram-negative bacilli in the catheter and urinary tract.