Splenic marginal zone lymphoma (SMZL) is generally an indolent disorder which has a median survival of 10 – 13 years, but a minority of patients pursue a more aggressive clinical course. A number of adverse prognostic factors including the level of haemoglobin, lymphocyte count and platelet count, B2 microglobulin, presence of a paraprotein, IgVH gene mutational status and p53 loss or mutation have been identified, but these have not been consistent among recent series. We have studied 89 patients with SMZL, diagnosed on the basis of lymphocyte morphology, immunophenotype and marrow and splenic histology, when available, and have evaluated the impact of presenting Hb, lymphocyte count, spleen size, presence of a paraprotein, cytogenetic abnormalities and IgVH gene mutational status on both the need for treatment and on overall survival. In 43 patients the diagnosis was made following a routine blood count performed for an incidental reason. The M:F ratio was 1:1.78, median age at presentation was 67.5 years (range; 49 – 91) and mean follow-up was 78.1 months. 63/89 (71%)patients presented with palpable splenomegaly, and a further 5 developed splenomegaly during the course of their disease. 42/89 (47%) patients required treatment of whom 29 (32%) underwent splenectomy. 36 patients have not required treatment, 25 have received an alkylating agent, 12 have received fludarabine and 4 were treated with rituximab.15/29 (17%) received chemotherapy for progressive disease post splenectomy. In six patients there was transformation to a high grade lymphoma. 14 patients (16%) died, 11 of whom had a disease-related death. 24/83 patients (29%) had a paraprotein, 67/87 (77%) had an abnormal karyotype of whom 39 (45%) had an abnormality of 7q. Using interphase FISH, 7/78((9%) showed trisomy 3 and 7/74(9%) had p53 loss. Of the patients with high grade transformation, 3/6 had p53 loss. 47/67 patients (70%) had mutated IgVH genes, and 15 cases utilised the VH1-02 gene segment. Only spleen size >10cm (p=0.0002) and Hb <100 gm/l (p=0.008) correlated with the need for treatment. Only unmutated IgVH genes (p=0.032) and the need for further therapy following initial treatment with splenectomy (p=0.034) correlated with overall survival.The median survival of unmutated cases was 113 months and was not reached for mutated cases.The median survival of patients for whom splenectomy has been their only treatment has not been reached, but was 110 months for patients requiring additional treatment post splenectomy. There is a need for larger, multi-centre studies to identify poor risk patients with SMZL.