The case of a 22-year-old man with polyneuropathy, endocrinopathy, skin change and monoclonal gammopathy of IgG-lambda type is described. There was no solitary plasmocytoma, osteosclerotic myeloma or Castleman's disease. However, significant thrombocytosis occurred and the patient developed arterial thrombosis, these were attributed to essential thrombocythaemia in the absence of other aetiological factors.