Affected children are usually born apparently normal and their condition worsens progressively without any possible treatment available to prevent this evolution. The clinical findings in alpha-mannosidosis include a broad range of symptoms, from an early lethal form to less symptomatic, chronic forms often initially diagnosed in childhood. Alpha-mannosidosis is frequently associated with corneal opacities, aseptic destructive arthritis, metabolic myopathy and immune deficiency. In the past alpha-mannosidosis was classified into two forms. A more severe infantile (type 1) phenotype that include rapid, progressive mental retardation; hepatosplenomegaly; severe dysostosis multiplex; and often death between 3 and 12 years of age. The juvenile-adult phenotype (type 2) is characterized by a milder and more slowly progressive course with survival into adulthood. The distinctions are not absolute, and symptoms and lethality may vary. In affected children that are born healthy, there is a window of opportunity for a therapy initiated at an early age to contribute to normal development and the prevention of other disease related complications.