Abstract The aim of the study was to determine folate reserve in patients with thyroid and breast diseases. Erythrocyte folate concentrations of ninety-eight patients and thirty healthy subjects were determined by a modified microbiological assay. An increase in folate levels was observed in of both thyroid (n = 15) and breast (n = 22) cancer patients compared to controls, while a decrease was observed in benign breast disease patients (n = 24) compared to both control group and breast cancer patients. Patients with benign thyroid diseases (n = 37) had significantly lower folate concentrations as compared to both control and thyroid cancer groups. The results pointed out the elevated erythrocyte folate levels in the patients with malignant tumors.