Background and Design: Isotretinoin has many adverse reactions such as cheilitis, xerosis, alopecia, hyperlipidemia, hepatotoxicitity and teratogenicity. Biotinidase is mainly produced in the liver and partial biotinidase deficiency causes dermatological manifestations, seborrheic dermatitis, alopecia etc. Some studies suggests that isotretinoin reduces the serum botinidase activity and some adverse effects might be related this deficiency. In addition, some literatures had reported that the teratogenic findings related hypervitaminosis A has similar characteristics the teratogenic findings related biotinidase deficiency. In this study, we aimed to investigate the effect of isotretinion on biotinidase activity in Turkish patients with acne vulgaris.