Urinary neopterin levels, blood dihydropteridine reductase activity as well as other frequently used clinical parameters were evaluated in 110 patients suffering from various types of lymphomas and leukemias. Among them neopterin was detected as the most sensitive marker representing the severity of malignancy (p<0.00001). All patients with active diseases had significantly raised urinary neopterin levels compared to those in remission and healthy controls. Of 69 patients with active disease 66 (96%) were above the upper limit seen in healthy subjects. In addition, the highest neopterin excretion was found in patients with active chronic myeloid leukemia (1469+/-479 micromol/mol creatinine n=16). In contrast, only 1 of 41 patients in stable responsive disease and remission (2.4%) had increased urinary neopterin levels above the upper limit. Dihydropteridine reductase (DHPR) activities were also detected in all patients and control groups. In active disease slightly reduced (DHPR) activities were evident (3.42+/-0.37 for controls, 2.92+/-0.39 in active disease and 3.28+/-0.42 nmol red cytochrome C/min/5 mm diameter disc in remission patients). However in patients under medication this was strengthened. This data also suggest that DHPR activity can be effected by chemotherapy. The results of the present study support the fact that urinary neopterin levels may be an useful and reliable early prognostic marker for neoplasia when used together with other prognostic indicators. Our data also suggest that reductions in DHPR activities may also be an underlying cause for the neurological disorders that are commonly seen in patients with haematological malignancies.