BACKGROUND: Topical retinoids normalize desquamation, reduce comedogenesis and may enhance the penetration of other topicals providing more effective treatment of acne. AIM: We evaluated the effect of adapalene on skin penetration of clindamycin phosphate when it is applied concomitantly or after various time durations following adapalene application. METHODS: The in vitro studies were carried out using excised rat skin, whereas the in vivo studies were conducted on healthy human volunteers. Radioactive clindamycin phosphate (1%) gel was applied to rat skin sections and to the hands of human volunteers concomitantly and after the pretreatment of the skin for 3, 5 and 10 min with 10 mg of adapalene (0.1%) gel. Quantification of clindamycin phosphate was performed by liquid scintillation. RESULTS: In vitro skin penetration and distribution of clindamycin phosphate was affected by the pretreatment time. Significantly higher skin concentration of clindamycin phosphate (15.5%) with largest proportion in viable skin layer (9.4% of applied dose) was found when clindamycin phosphate gel was applied after the pretreatment of the skin with adapalene gel for 5 min. Further increase in pretreatment time has no additive influence on the penetration of clindamycin phosphate. In vivo results were in corroboration with the in vitro results and demonstrate significantly higher concentration of clindamycin phosphate (19%) in the skin following pretreatment with adapalene gel for 5 min. Adapalene acts as a penetration enhancer and increases the penetration of topical clindamycin phosphate. CONCLUSION: Application of clindamycin phosphate gel after the pretreatment of skin with adapalene gel for 5 min may contribute significantly to the increased efficacy of therapy.