Between May 84 and December 85, 61 patients were treated for hairy cell leukemia by (recombinant or natural) alpha-IFN in a single institution. The responses obtained after 7 or 12 months of treatment are presented and the follow-up of the 39 patients for whom the treatment had been stopped for more than 12 months is evaluated. Neither clinical or histological parameters at presentation nor the type of timing of the interferon used allowed to predict the quality of response. The median time-lapse before the occurrence of neutropenia below 1.10(9)/l in patients with normal blood counts at the end of treatment was 12 months. However early relapses after the treatment stop were observed more frequently in patients with initially severe neutropenia or hyperleucocytosis. The therapeutic strategy in hairy cell leukemia is discussed.