Clinical heterogeneity in MPS IV A (Mucopolysaccharidosis IV A, Morquio Disease Type A)has become more clearly identified in recent years. The clinical findings in 12 cases of MPS IV A are described. Clinical presentation was variable, and some cases were only mildly affected. All showed deficiency of N-acetylgalactosamine-6-sulphate sulphatase in fibroblasts, but the patient with the mildest clinical presentation showed a high residual enzyme activity. The urinary glycosaminoglycans (GAGs) were examined on all patients by a two-dimensional electrophoresis technique which proved to be highly reliable and efficient. In particular, no false negative results were obtained, a problem often encountered with routine screening methods. These cases support the division of MPS IV A into three subgroups: the severe "classical" type, an intermediate type and a mild type, all caused by N-acetylgalactosamine-6-sulphate sulphatase deficiency. Residual enzyme activity may be an important prognostic indicator.