The tar, nicotine, and carbon monoxide yields of cigarettes manufactured in the United Kingdom between 1934 and 1979 were studied. Over this period the average tar yield decreased by 49%, the nicotine yield by 31%, the carbon monoxide yield by 11%, all estimated on a sales-weighted basis. The average tar yield decreased progressively after the second world war, owing both to the introduction of filter cigarettes and to changes in the manufacture of plain cigarettes. The average nicotine yield increased initially, decreased by 43% from about 1950 to 1974, but increased again by 9% between 1974 and 1979. The average carbon monoxide yield started to decrease after about 1961; while it decreased substantially in plain cigarettes, the rapid increase in sales of filter cigarettes at this time, at the expense of plain cigarettes, largely offset the reduction in carbon monoxide yield that would otherwise have occurred. As with nicotine, carbon monoxide yield showed a small rise in later years (4% between 1976 and 1979). The trends in tar yield may well explain the reduction in lung cancer in the UK better than has been suspected hitherto. The trends in nicotine and carbon monoxide yields are probably not sufficiently different to distinguish which of them might be the more likely cause of cardiovascular disease.