The clinical significance of low serum vitamin B12 levels in elderly people is controversial. We aimed to document the prevalence of a low serum vitamin B12 (<175pmol/l) in patients referred to a geriatric medical unit, and to determine whether haemopoiesis is commonly affected in elderly patients with low serum vitamin B12. We studied prospectively 472 consecutive referrals to a geriatric medical unit; fifty-six (13%) had a low serum vitamin B12 level, of whom nineteen (34%) of the fifty-six also had evidence of Fe deficiency (serum ferritin<45ng/ml). Low vitamin B12 was associated with a raised mean erythrocyte volume (MCV; mean 96·0 (SD 6·7) fl), compared with a control group (91·7 (SD 6·0) fl; P=0·001). However, only thirteen (23%) of the fifty-six patients with a low vitamin Blz had an MCV≥100 fl. Mean haemoglobin (Hb) levels were not significantly reduced in those with a low vitamin B12. In a subsequent study the haematological response to intramuscular hydroxocobalamin was examined in thirty-four patients with a low serum vitamin B12. Treatment resulted in a significant fall in MCV and rise in Hb; these effects could be detected both in those patients with an initially normal full blood count (change in MCV -1·2 (SD 1·2); Hb + 0·5 (SD 0·6); P<0·01) and in those with macrocytosis and/or anaemia (-9·1 (SD 11·8); + 0·8 (SD 1·2); P<0·05). A low serum vitamin B12 is common in geriatric medical patients. This is usually associated with an upset in erythropoiesis, although the abnormalities are often subtle and may not be apparent on inspection of the full blood count. Elderly patients with serum vitamin B12<175pmol/l should be assumed to have vitamin deficiency even if their full blood count is normal.