Alpha-mannosidosis is a rare autosomal recessively inherited lysosomal storage disorder. We describe three patients with alpha-mannosidosis who were born in Tromsø between 1983 and 1987, in order to increase awareness of the disease. It is characterized by a typical facial look, with a prominent forehead, hypertelorism, small nose, flat nasal bridge and hypoplastic teeth. The patients are mentally retarded, often have dysostosis multiplex, recurrent infections and typically severe loss of hearing and delayed speech development. The disease is slowly progressive in the first decade, but shows considerable clinical variability. In most cases, the lymphocytes are vacuolized, but diagnosis depends on measurement of alpha-mannosidase activity in the lymphocytes. Prenatal diagnosis is available, based on chorionic villi sampling in the 9th to 11th week of pregnancy. No causal therapy is known, but establishment of the diagnosis is important to avoid complications, recognize hearing loss and provide speech therapy and special education. The specific diagnosis is critical for genetic counselling and prenatal diagnosis. The authors therefore outline the diagnostic strategy.