Most alpha-mannosidosis patients described have been children and information on the natural course of the disorder has been based on a very limited number of observations. In order to assess the disease presentation in detail and to study disease characteristics, a study was started in 1991 and has been ongoing for over 20 years. Patients with confirmed alpha-mannosidosis were recruited through The International Society for Mannosidosis and Related Diseases (ISMRD) where families affected with alpha-mannosidosis received a questionnaire on general clinical information to be filled out by the responsible physician. The questionnaire was returned by 125 patients (64%). Of these, 45 patients were 15 years old or older at the time of evaluation. The questionnaire allowed us to assess the following features: Facial dysmorphism, columnar disease, arthritis, myopathy, hearing impairment, mental impairment, psychosis, bone disease and motor function as well as general health. This study describes the progression of alpha-mannosidosis and may be helpful in determining the clinical characteristics for assessments of prognosis.