Chronic myeloid leukemia (CML) is usually treated with either alpha-interferon or hydrea. Median survival is 6 years. Eventually, in most CML patients, the disease evolves to blast phase with clinical and morphologic characteristics of an acute leukemia. This phase is commonly associated with systemic symptoms and the appearance of new cytogenetic abnormalities. Therapy for this phase is of limited value, resulting in a mean survival of 4 months. We describe four consecutive patients seen at our clinic with advanced stage CML (three blast, one accelerated phase) who were treated with interleukin-2 (Proleukin). The mean survival in these patients was 22 months (range 9-35 months) and two are still alive 25 and 35 months after the start of therapy. One patient had a complete cytogenetic response and another a partial response. Toxicity was minimal and no patient had to discontinue therapy because of it.