In 93 patients suffering from prostatic cancer, serum levels of neopterin, a pteridine reflecting activation of the immune defence system, were measured at the time of diagnosis and 6 and 12 months after treatment with orchidectomy or estrogens. Neopterin levels were initially greater than 10 nM/l in 27% of the patients and in this group tumor recurrences occurred in 28% as compared to 7% in the group with neopterin less than 10 nM/l. No apparent correlation between tumor stage and grade and neopterin levels could be demonstrated. Neopterin levels decreased significantly after onset of treatment (p less than 0.001), which implicates a hormone dependence. But still patients with initially elevated serum neopterin levels, and thus an assumed activated immune defence system at the time of diagnosis, had lower survival rates than those without elevated neopterin levels (p less than 0.001). It is concluded that elevated levels of neopterin in serum is a sign of poor prognosis in human prostatic cancer.