Eighteen patients with urinary tract infection were treated with cephalexin orally. Absorption was variable, between 29 and 89% of the total daily dose being excreted in the urine in 24 hours. A significant number of patients became faecal carriers of Pseudomonas aeruginosa compared with a control group who received no antibiotics. Four of the cephalexin-treated patients acquired a strain of Ps. aeruginosa known to be present in food from the hospital diet kitchen and one developed a urinary tract infection with this strain.