We report on two unrelated patients with different presentations of mannosidosis. One patient was affected in early childhood with a severe phenotype characteristic of type I mannosidosis. The other was diagnosed with type II mannosidosis only after the onset of progressive neurologic deterioration in late adulthood. Both were detected by non-invasive urinary screening of oligosaccharides. Lymphoblasts transformed from both patients' blood cells had markedly reduced lysosomal alpha-mannosidase activity. Kinetic analyses showed that alpha-mannosidase from the type I patient had a 400-fold reduction in affinity while that from the type II patient was reduced 40-fold. Lymphoblasts from all 4 parents had reduced alpha-mannosidase activity, but there were overlapping activities among these type I and type II obligate heterozygotes. We conclude that screening urinary oligosaccharides will detect mannosidosis over a wide range of phenotypes, that lymphoblasts transformed from affected heterozygotes have decreased enzymatic activity, and that the severity of clinical expression is related to the degree of enzyme impairment.