The usefulness of hypochromia (MCH <or = 26 pg) and microcytosis (MCV < or = 80 fl) as indicators of iron deficiency was examined in 365 geriatric patients aged 67-96 years with either one or both or these characteristics. Of these, 201 proved to be iron deficient with a serum ferritin of 18 microg/l. There was a highly significant difference (P < 0.001) between the proportion of iron deficient patients with a mean corpuscular haemoglobin (MCH) < 26 pg (in the presence of a mean corpuscular volume (MCV) > or = 80 (fl) vs. counterparts with MCV < 80 fl (in the presence of MCH > or = 26 pg). Fifteen per cent of the 201 iron deficient subjects were also shown to have coexisting vitamin B12 deficiency. There was a comparable (16%) prevalence of this haematinic deficiency in the subgroup of 31 iron deficient patients with MCH < 26 pg in the presence of MCH > or = 80 fl.