The number of mitoses per high power field (mitotic index, MI) was assessed in 2 microns sections of lymph node biopsies from 58 adults with non-Hodgkin's lymphoma. All had diffuse nodal lymphomas of unfavourable histology and stage II-IV disease. The patients were treated with chemotherapy and followed for a minimum of 3 years or until death. None out of 29 patients with a MI greater than or equal to 3.0 survived for 3 years after diagnosis whereas 13 out of 29 other patients with MI less than 3.0 became long-term survivors (P = 0.00002). Differences in age, sex or clinical stage between short- and long-term survivors were negligible. The initial chemotherapy regimens were not more intense for the long-term survivors. Twenty-nine patients were given an equivalent initial treatment with CHOP or CHOP plus methotrexate. The association between MIs and survival was evident also in this subgroup. The results indicate that survival is extremely poor for patients with advanced diffuse nodal lymphomas of unfavourable histology and a high mitotic count. It seems especially important to evaluate alternative chemotherapy regimens, suggested to be more effective than current programmes, in this subset of patients.