Clindamycin has become a highly popular drug for the topical therapy of acne; however, the extent to which it is systemically absorbed from the skin has has not been established. We measured the serum level and urinary excretion of clindamycin on the third day and the twenty-seventh day of therapy in thirteen patients who were applying 1% clindamycin hydrochloride topically for acne. There was no detectable antibiotic in the serum of any subject (less than 0.4 microgram/ml); in contrast, clindamycin was found in the urine of ten of the thirteen patients. There was marked intersubject variation in the urinary excretion of the drug, ranging from less than 10 to 500 micrograms/day. However, there was a highly significant correlation (p less than 0.0001) for a given subject between excretion values on days 3 and 27. There was no correlation between urinary excretion of clindamycin and either racial pigmentation or severity of acne in this relatively small group of patients. After topical application of 1% clindamycin hydrochloride, an average of 4% to 5% of clindamycin appears to be absorbed systemically, but greater amounts are absorbed in some individuals.