The incidence of and factors associated with diarrhea were evaluated in 1,000 patients receiving clindamycin. Diarrhea occurred in 6.6% of the patients, and of the many parameters evaluated, a significant association with diarrhea was found only for age (higher incidence in patients older than 20 years), sex (higher incidence in females), and route of administration (higher incidence with oral than with parenteral administration). It is of interest that we were unable to find a positive correlation between dose of drug or duration of therapy and diarrhea. Diarrhea began within 48 hr of start of clindamycin therapy in 52.3% of the patients and within five days of therapy in 75.3%. Nineteen patients developed diarrhea after they had stopped receiving the drug. Duration of diarrhea varied considerably, with a mean of 11.4 days and a range of one to 120 days.