Three hundred and fifty-eight stool and 131 sputum specimens from 40 cystic fibrosis (CF) patients and 100 toilet sinks were investigated for occurrence of Pseudomonas aeruginosa; 67% (21/31) of the patients with chronic P. aeruginosa lung infections carried the organism repeatedly in the stool but the organism was found only once in the stools of nine uninfected patients. P. aeruginosa stool carriage was correlated to high P. aeruginosa numbers in patients' sputa. Typing of P. aeruginosa with a DNA probe showed identity of sputum and stool strains. Seven patients repeatedly carried additional stool strains, not found in the sputum, suggesting intestinal colonization. No differences were seen in the clinical state of patients with P. aeruginosa-negative stool samples and patients with positive stool samples. Toilets in households of P. aeruginosa-infected CF patients were significantly more often contaminated with P. aeruginosa (42%) than toilets in households of non-infected CF patients (20%; P less than 0.03). The study shows that P. aeruginosa-infected CF patients may harbour the organisms also in the intestinal tract, and may spread the bacteria into toilets.