During 1966 and 1986 202 patients with hairy cell leukemia in the bone marrow biopsy were observed. All hairy cells were categorized according to their nuclear morphology into ovoid subtype, convoluted subtype and indented subtype. The overall survival of 113 non-splenectomised patients was 11 months. However, the median survival time of each subtype differed considerably with 55 months for the ovoid subtype, 8 months for the convoluted and 6 months for the indented type. The prognosis of splenectomy patients were significantly better at a p-value of less than 0.0001 than the survival of the non-splenectomised patients. The median survival time was 59 months. Also in this group of patients the prognosis was different for the ovoid, convoluted and indented subtype. The median survival time of the ovoid subtype is not yet reached, despite a observation time of more than 180 months. In contrast, the survival time for the convoluted and indented subtypes was 26 months. These data elucidate that at least part of the heterogeneity in the clinical course of this disease can be explained by the morphologically distinct subtypes and that splenectomy prolongs profoundly but to a different degree the survival of patients with all three histological subtypes. The response rate to r.-IFN alpha-2b is higher in patients with the ovoid than with the convoluted and indented subtypes.