: OBJECTIVES: Although there is increasing evidence that the pineal gland may play a role in human malignancy, the studies on melatonin concentrations in different types of malignant tumors brought about controversial results. However, changes in melatonin concentrations have been observed in some types of human malignant tumors. Therefore we decided to study the circadian melatonin rhythm in patients suffering from malignant tumors of the female genital tract, and to compare them with subjects free from neoplastic disease (healthy volunteers and patients with myomatous uterus). MATERIAL AND METHODS: A total of 46 women were analyzed in this study. The subjects were divided into 3 groups. The first group consisted of 23 patients with malignant tumors of the genital tract (mean age 50.3+/-2.2 years; mean+/-SEM, range 32-77 years). The second group consisted of 16 healthy volunteers (mean age 50.9+/-1.8 years; mean+/-SEM, range 42-63) who served as the first control group, whereas the third group consisted of 7 subjects who suffered from myomatous uterus (mean age 45.7+/-2.3 years; mean+/-SEM, range 39-56) and served as the second control group without malignancy. Blood samples were collected at 08:00, 12:00, 16:00, 20:00, 22:00, 24:00, 02:00, 04:00, 06:00 and 08:00 h. Melatonin concentration was measured using RIA kit. RESULTS: There were no significant differences in circadian melatonin profiles among the three groups studied. Taking into consideration the type of tumor of the genital tract, significantly lower melatonin secretion has been found in patients with endometrial cancer in comparison with tumor-free control groups, whereas no significant differences in melatonin secretion have been observed between tumor-free control groups and patients with invasive ovarian cancer and squamous cervical cancer. However, significant differences have been observed between endometrial cancer and invasive ovarian cancer. CONCLUSION: Its seems probable that melatonin concentrations in human malignancy may, at least partly, depend on hormone dependency of the particular type of tumor.