The serum levels of the pineal hormone melatonin were determined by radioimmunoassay (RIA) in 4-h intervals throughout a 24-h period in elderly men with different types of prostate tumors: benign prostatic hyperplasia (BPH, n = 13), incidental carcinoma (PCi, n = 5), and nonmetastasizing carcinoma (PC, n = 9), as well as in young men (YM, n = 10). Simultaneously, the pituitary hormones prolactin, growth hormone, luteinizing hormone and follicle-stimulating hormone were measured by RIA. All subjects were untreated and free of serious complaints, and they stayed in the same environment. The data were analyzed by the population mean-cosinor method, and linear correlation coefficients between the five hormones were calculated for each group. Melatonin showed significant circadian rhythms in young men and patients with BPH and PCi but not in patients with PC. Twenty-four-hour mean concentration (mesor) and amplitude were significantly increased in patients with PCi as compared to patients with PC. Prolactin showed significant circadian rhythms in young men and in patients with BPH, whereas patients with PCi and PC appeared to have ultradian variations. Growth hormone did not show significant rhythms in any of the groups; the mesors were elevated in all tumor groups as compared to young men. Gonadotropin mesors were elevated in all tumor patients as compared to young men; rhythms were not detected. Carcinoma patients showed different interhormonal correlations than all other groups. These results indicate that modulation of melatonin secretion, accompanied by changes in the pituitary hormone levels, may be related to development and growth of prostate cancer.