Splenic lymphoma with villous lymphocytes (SLVL) is a low-grade B-cell lymphoma defined in the World Health Organization classification as the leukaemic form of splenic marginal zone lymphoma. Presenting features and response to therapy have been described, but information on prognostic factors is scanty. Clinical, laboratory and follow-up data were collected on 129 patients with SLVL to determine features predicting disease behaviour and survival. Diagnosis was made on clinical, morphological and immunophenotypic features and, where available, bone marrow and spleen histology. Median age was 69 years (range 39-90 years) and male:female ratio, 0.9. The majority had splenomegaly, but lymphadenopathy and hepatomegaly were rare. Median Hb was 11.8 g/dl, white blood cell count was 16 x 10(9)/l and platelet count was 145 x 10(9)/l; 27% of patients had monoclonal protein in serum and/or urine. While 27% of patients remained untreated, 10% transformed to high-grade lymphoma. Median follow-up was 61 months and median survival was 13 years, with 72% of patients alive at 5 years. Cox regression analysis showed that increasing age, anaemia, thrombocytopenia and lymphocytosis > 16 x 10(9)/l were independent adverse predictors of overall survival. However, only anaemia and lymphocytosis > 16 x 10(9)/l remained highly significant independent prognostic factors when only deaths due to lymphoma were analysed. Splenectomized patients fared better than those receiving chemotherapy only (P = 0.001 for SLVL deaths). We conclude that SLVL is mainly a disease of the elderly with a relatively benign course but, when treatment is required, splenectomy is beneficial.