Urinary neopterin levels were studied in 96 patients with malignant lymphomas. Twenty-eight had Hodgkin's disease and 68 non-Hodgkin's lymphoma. Neopterin excretion was significantly related to the clinical stage of the disease. Mean neopterin excretion in patients with active disease (634 +/- 527 mumol neopterin/mol creatinine) was significantly higher (p = 0.000) than in patients in complete remission (198 +/- 105 mumol neopterin/mol creatinine). Mean neopterin levels of patients in stage III-IV were higher than for patients in stage I-II. These findings were the same in patients with Hodgkin's disease and those with non-Hodgkin's lymphoma (659 +/- 593-425 +/- 316 mumol neopterin/mol creatinine), regardless of the histological subtype. A significant correlation was found between neopterin excretion, ESR (r = 0.31; p = 0.003) and hemoglobin (r = -0.40; p = 0.000). Longitudinal analysis showed a trend towards a correlation between response to therapy and neopterin excretion. These findings suggest that neopterin may be a useful prognostic marker in non-Hodgkin's lymphoma.