Interferon-alpha combined with retinoid or PUVA is used for the treatment of cutaneous T-cell lymphoma. Anti-IFN-alpha antibodies (IFN ab) occur regularly during IFN-alpha treatment. We investigated the incidence of neutralizing and binding IFN ab and analysed their relationship with clinical and immunological parameters. A group of 17 CTCL patients were treated with IFN alpha-2a three times weekly subcutaneously at a dose of 3 Mill. I.U. combined either with retinoid (acitretin, Neotigason; 0.5 mg/kg bodyweight) daily or with 5-methoxypsoralen (1.2 mg/kg bodyweight) plus UVA radiation three times weekly. Prior to and during treatment we monitored stage, skin involvement by a tumour burden index, serum levels of beta 2-microglobulin, neopterin, binding and neutralizing IFN ab, Interleukin-6 (IL-6), soluble IL-2 receptors (sIL-2r) and the CD4/CD8 ratio of peripheral blood mononuclear cells. We observed two complete, two partial and six minor responses, four patients with stable disease and three patients with progressive disease. Of the 17 patients, 7 developed binding IFN ab, but only 2 had neutralizing IFN ab which were associated with high titres of binding IFN ab. IFN ab formation was more frequent in patients with normal CD4/CD8 ratios and a high tumour burden index and showed a trend to be more frequent in PUVA-cotreated patients than in retinoid-cotreated patients. Responses were more frequently seen in IFN ab-negative patients. IFN ab developed in patients treated with PUVA or retinoid combined with IFN. Binding as well as neutralizing IFN ab may have an impact on the treatment success in CTCL patients.