Lactose maldigestion has been under intensive research since its discovery in the 1960's. We know the prevalence of lactose maldigestion in a great number of countries and ethnic groups. However, there is often no provision made for the secondary type of maldigestion, and the study populations have sometimes been selected rather than picked at random. New methods for the measurement of lactose digestion have been developed, and its genetic mechanisms have received a great deal of attention during the last few years. However, in many studies the measurement and/or reporting of symptoms has quite often been overlooked. In this review, various topics related to lactose intolerance are discussed with a special emphasis on its symptoms.