Cyclosporin A (CyA) is a fungal peptide used as immunosuppressor in human organ transplantation. Increased hair growth appears to be a common side-effect of treatment, occurring in almost all patients treated with oral CyA. The development of hypertrichosis during CyA administration is not hormone-dependent; it may be mediated by a direct effect of the drug on the hair growth process. We used CyA topically (5 p. 100 oily solution) in 14 patients with severe alopecia areata (totalis 5, ophiasis 3, bald plaques 6). The drug was applied on the alopecia twice a day for 5 +/- 1.5 months. Patients were examined monthly. No clinical adverse effect was observed. Routine blood examination did not show any change, except for a transient increase of hepatic enzymes in one patient. CyA determinations were performed monthly by radioimmunoassay. No CyA was detected in the patients' serum. Treatment resulted in no regrowth in 8 patients, development of vellus hair in 3 patients (alopecia areata totalis 2, ophiasis 1) at the end of therapy, and normal hair growth in 3 patients with bald plaques. We may conclude that CyA is capable of inducing regrowth of hair when applied topically. However, the drug does not seem to be useful for the treatment of alopecia areata.