The case of a 22-year-old man with polyneuropathy, endocrinopathy, ski
n change and monoclonal gammopathy of IgG-lambda type is described. Th
ere was no solitary plasmocytoma, osteosclerotic myeloma or Castleman'
s disease. However, significant thrombocytosis occurred and the patien
t developed arterial thrombosis, these were attributed to essential th
rombocythaemia in the absence of other aetiological factors.