In 520 men who currently smoked only cigarettes, carboxyhaemoglobin (COHb) levels were measured as a method of estimating the extent to which cigarette smoke was inhaled and the results were compared with the smokers' own assessment of their inhaling habits. The mean COHb level after standardising for the number of cigarettes smoked before the blood test on the day of the test was 4.0% in self-described non-inhalers. This was much higher than the mean level of 0.7% in 1891 similar non-smokers, but not very different from the standardised mean levels of 5.2%, 5.3%, and 5.6% in men who said they inhaled slightly, moderately, or deeply, respectively. The increasing trend in the COHb levels of men in the four self-described inhaling categories (nil to deep) was small but statistically highly significant. The data from this study may help to explain some of the anomalous epidemiological results regarding the relationship between self-described inhaling habits and the development of diseases associated with smoking, such as coronary heart disease and lung cancer.