In this paper nine patients with mucolipidosis II (I-cell disease) are described. They had clinical features commonly found in mucolipidosis II, including disproportionate dwarfism, coarse facial features and mental retardation. However, there was remarkable variability in age of onset, organ manifestation and radiological findings. Some had unusual clinical symptoms including pericardial effusion and profound brain atrophy. Striking differences in phenotypic expression were also seen in two affected siblings. Clinical heterogeneity is observed not only in mucolipidosis II but also in many other lysosomal storage disorders. The factors that may contribute to this clinical diversity are discussed.