Lactose is a disaccharide consisting of a glucose and a galactose. Milk is a sole source of lactose in natural food. Lactase hydrolyzes lactose into glucose and galactose, which are absorbed in the intestine and utilized by the body. After weaning, lactase activity declines. Therefore, the majority of adults have hypolactasia. The prevalence is above 50% in South America, Africa, and Asia. A significant amount of lactose is consumed as cow milk-based food products. However, it has no special nutritional importance for adults, and it is suspected as a dietary risk factor for coronary heart disease (CHD) and ovarian cancer. The aim of this study is to review the evidence and to propose possible mechanisms regarding the relations between lactose intake and CHD and ovarian cancer. Epidemiological studies support the connection of lactose and CHD. Although several mechanisms for elucidation of this connection have been suggested, calcium theory is the most plausible. Galactose is known to be toxic to ovary. Galactose has been suggested to induce ovarian cancer through hypergonadotropic hypogonadism. Some epidemiological data support this but others do not. Consumption of lactose or galactose can induce CHD and ovarian cancer; however, more research will be needed in order to determine the relation between lactose and these diseases.