The carrier rate of Clostridium difficile in an adult Swedish population was found to be 11 (1.9%) of 594. All isolates were toxigenic in vitro, but no healthy individual harbored free cytotoxin in stool. Of 398 patients with acute diarrhea not associated with antibiotic use, cytotoxin was found in stool filtrates of four (1010). In 4,793 patients with antibiotic-associated diarrhea from all parts of Sweden during 1980–1982, C. difficile cytotoxin was demonstrated in 873 (18%). The tissue culture assay was found to be more specific than cultivation for the bacterium. By weighted analysis, in the age group >70 years more women than men were infected. In the age group 21–50 years there was an even greater preponderance of infection in women than in men. Cephalosporins and lincosamides were 10–70 times more often implicated in C. difficile colitis than were narrow-spectrum penicillins.