A prospective survey of the level of haemoglobin (Hb) has been carried out in 1057 subjects (607, 57.4% women) over the age of 40 randomly selected from the age-sex register of a North London group practice. Mean Hb levels were 15.04 +/- s.d. 1.41 g/dl in men and 13.54 +/- s.d. 1.32 g/dl in women (P less than 0.001). In men, Hb levels did not differ between the ethnic groups represented, but women of Mediterranean origin had significantly lower Hb levels than other women (P less than 0.001). There was no social class difference in Hb levels. Women under age 50 had significantly (P less than 0.001) lower Hb levels than older women. Smokers had significantly higher Hb levels than non-smokers (P less than 0.002) and there was a weak correlation with numbers of cigarettes smoked (men rs = 0.07, P = 0.07; women rs 0.14, P less than 0.001). Haemoglobin levels correlated with diastolic blood pressure in both men (r = 0.07, P less than 0.05) and women (r = 0.23, P less than 0.001). Thirty eight subjects (3.6%), 25 women and 13 men, were found to be anaemic (Hb less than 11.5 g/dl in women or less than 12.5 g/dl in men). Anaemia could be confirmed in only six of 19 subjects in whom repeat levels were measured. Screening for anaemia did not uncover any serious disease.