Mast cells have been counted in iliac bone biopsies from patients with chronic renal failure treated by haemodialysis, and attempts made to relate mast cells to the various features of azotaemic renal osteodystrophy. There was a significant increase in the number of marrow mast cells in comparison with controls, and haemodialysis has no favourable effect on the marrow mast cell hyperplasia which occurs in chronic renal failure. There was a general tendency towards greater numbers of mast cells in cases with most marked ostetis fibrosa but it was not possible consistently to relate hyperparathyroidism to mast cell numbers. Patients treated by parathyroidectomy, aluminium hydroxide, or the newer vitamin D metabolites failed to show any consistent fall in marrow mast cell numbers in spite of improvement in the ostetis fibrosa. The number of marrow mast cells was related to the duration of haemodialysis and to the amount of osteoid present and iversely to the percentage mineralization of bone. It was not possible to identify any causal relationship between the presence or severity of osteomalacia and the number of mast cells in the haemodialysed patients although mast cell hyperplasia was also observed in patients with osteomalacia unassociated with renal disease.