Longitudinal assessments of three brothers with alpha-mannosidosis were performed biochemically by determining levels of leukocyte enzyme activity, and neurodevelopmentally by testing of general intelligence, language, visual spatial skills, and overall adaptive abilities. During the follow-up examination, enzyme activity was assessed in fibroblasts to evaluate the uniformity of biochemical deficits. The biochemical findings demonstrated profound deficits of leukocyte alpha-mannosidase that remained remarkably stable over time and were very similar to levels of the same enzyme activity in fibroblasts. The cognitive findings showed that the patients manifested mild cognitive deficits. Cognitive deficits were generally uniform with no signs of progressive deterioration, except receptive language abilities. Suggestions are made for careful follow-up of auditory abilities in patients with mannosidosis.