Galactosemia is an inborn error of metabolism, caused by the deficiency of three of four enzymes involved in galactose metabolism (GALK, GALT or GALE). Impaired breakdown of galactose results in accumulation of galactose and its metabolites in the tissues, which has harmful effects on different cell types, especially on lens, ovarian and brain cells. The most common error of galactose metabolism is GALT deficiency. Two forms of GALT deficiency galactosemia exist: classical and Duarte 2 galactosemia, first with complete and second with partial loss of enzyme activity. The most common classical galactosemia mutations are Q188R and K285N. Mutations N314D, IVS4-27G>C, IVS5+62G>A, IVS5-24G>A and -119_-116delGTCA are characteristic for Duarte 2 galactosemia. Ovarian cancer is the sixth most common type of cancer in women. It has the highest mortality rate among gynecological cancers. The causes for ovarian cancer have not yet been completely identified. It is probably caused by a combination of genetic and environmental factors. Studies have shown that the substances, toxic to oocytes, can induce ovarian cancer. It is therefore possible that galactose and its metabolites have the same effect. Individuals who frequently consume milk and milk products and those with high lactase activity in the adult age are chronically exposed to high concentrations of galactose and could be at a higher risk for developing ovarian cancer. We investigated the influence of low GALT and GALE activity, milk consumption and lactase status on ovarian cancer risk in 60 patients with ovarian cancer and 79 healthy women. We determined the mean GALT and GALE activities, the frequencies of the most common GALT gene mutations and the frequency of polymorphism C/T-13910 in both groups. We used DNA sequencing to search for new mutations in GALT and GALE gene in individuals with low activity of these enzymes. Milk and milk product consumption was assessed using a questionnaire. The mean GALT and GALE activities were significantly lower in the group of patients with ovarian cancer compared to the control group, while the frequencies of GALT gene mutations and C/T-13910 polymorphism did not differ significantly between the two groups. No new mutations were discovered in the GALT gene. In the GALE gene, we discovered one mutation in 5' UTR (g.404C>T), 4 intron mutations (IVS4+168C>A, IVS4+186G>C, IVS6+118C>G and IVS10+13G>A), 4 mutations that affect amino acid sequence of the enzyme (A84S, A89S, A254T and S312N) and two mutations in 3'UTR (g.5034G>A and g.5185-5187insTAA). These mutations could be the reason for low GALE activity. Consumption of whole milk was significantly higher in ovarian cancer group, than in control group. The risk for ovarian cancer is increased by impairment of GALT and GALE and by consumption of whole milk, especially in individuals with high lactase activity in the adult age.