The study was designed to test the ability of sequential applications of biotin-containing ointment to increase serum biotin levels. Twenty atopic dermatitis patients (mean age, 20.5 yr) and 11 healthy subjects (mean age, 25.5 yr) volunteered to participate in this study. The diagnosis of atopic dermatitis was established dermatologically. Seven grams per day of ointment containing 0.3% biotin and 1-4 g per day of steroid ointment were both applied sequentially. The healthy subjects applied only biotin ointment. The biotin concentration was determined microbiologically. Before biotin treatment, the average serum biotin level was significantly lower in atopic dermatitis patients than in healthy subjects. The percutaneous application of biotin-containing ointment caused a significant increase in the serum biotin concentration in both healthy subjects (from 41.5 +/- 10.0 to 50.2 +/- 9.2 nmol/L) and in atopic dermatitis patients (from 27.9 +/- 17.4 to 50.7 +/- 21.6 nmol/L), especially in patients whose initial level was low, and also could be effective in regulating the atopic allergic response involving eosinophils. In conclusion, biotin appears to be readily absorbed through both normal and dermatitis-affected human skin.