The aim of the present study is to investigate whether the values of CA 12-5 which is considered a specific ovarian tumor marker can be used in patients with non-gynecological tumors whose values in menstrual cycle phases were not previously investigated. In order to determine whether a particular phase or a combination of phase are important, CA 12-5 values were determined by radioimmunoassay technique in a limited number of patients belonging to three different age groups. CA 12-5 were found to be unrelated to age. No statistically significant difference in CA 12-5 values was noted between samples obtained from patients with tumor and healthy women on the first and second days of menstruation. In both cases the normal value exceeded 35 U/ml. The mean values was 42 U/ml in healthy women and 49 U/ml in tumor patients. Mean CA 12-5 values determined between the 12th and 14th days on which estrogen hormone is at its highest level, were found to be 23 U/ml in healthy women and 40 U/ml in patients with tumors. These values are statistically significant, while CA 12-5 values determined in samples obtained on the 20th-25th days are within the normal range. Values of the CA 12-5 tumor marker during the estrogenic phase are important in the diagnosis, management and follow-up of cancer. Determination of estrogen hormone levels as an additional parameter may provide a significant correlation.