Data are presented on the incidence of ischaemic (coronary) heart disease and duodenal ulcer among the several thousand male medical practitioners aged 35-64 holding immediate sickness benefit policies with the Medical Sickness Annuity and Life Assurance Society Limited. Three periods are considered: 1947-50, 1957-60, and 1961-5.The incidence of first clinical episodes of ischaemic heart disease in the doctors altered little between 1947-50 and 1957-60 but increased in 1961-5. Comparison of the late 1940s with the early 1960s shows a 60% rise of incidence at ages 45-54 but little change at other ages. Cases first presenting as "sudden" death increased between 1947-50 and 1961-5 by 111% at 45-54, and again changed little at 55-64. In two other occupational groups that have been studied-bus conductors and insurance salesmen-the increase of incidence was greater than for the doctors at 45-54 and it occurred also over 55 years of age. The increase from 1947-50 to 1961-5 in mortality during all episodes of ischaemic heart disease was the same in the doctors as in the male population of England and Wales at 45-54, but at 55-64 it was less.The results in the doctors are not due to alterations over the period in length of sickness absence, or underwriting policy, or of the nomenclature used on the certificates.Well-documented changes in the smoking habits of doctors may be partly responsible for what appears to have been a relatively favourable experience of ischaemic heart disease from 1947-50 to 1961-5, especially at ages 55-64.Incidence of duodenal ulcer at ages 35-64 declined steadily in this population of doctors from 1947-50 to 1961-5. The decline is very likely to be real.