AbstractMultiple sulphatase deficiency was studied in 3 siblings — one pair of monozygotic twins and their sister. The children's psychomotor development was arrested at the age of 18 to 24 months, and the hypotonic syndrome combined with signs of spasticity appeared. There was marked hepatosplenomegaly, conspicuously dry scaly skin with the decortication syndrome developing and persisting in the presence of pronounced cachexia. Also present were numerous X‐ray abnormalities, metachromatically staining granules in the urine, and Alder‐Reilly's bodies in the blood leukocytes and in specimens of bone marrow. Liver, skin and muscle biopsies performed simultaneously revealed accumulations of water‐soluble mucopolysaccharides and deposits of sulphatides in the two twins. Enzyme assays demonstrated arylsulphatase A and B deficiency. The diagnosis was subsequently confirmed at all the three siblings' postmortem examinations.