Recording of smoking status in general practice was compared according to the presence or absence of four diseases in which smoking cessation is important (asthma, hypertension, ischaemic heart disease and diabetes). Five of seven practices in one locality of Leicester took part. Our aims were to discover whether patients with these diseases were more likely to have a record of smoking status in their general practice notes than patients without them, and to examine the accuracy of this record compared with results from a patient questionnaire. For all diseases included, the proportion of records containing data on smoking status was higher if the disease was present than if it was not. When adjusted for age and sex, differences in recording reached statistical significance for asthma, diabetes and ischaemic heart disease, but not for hypertension. Smoking rates, as estimated from both the GP record and the postal questionnaire, were lower for all disease groups, but for asthma this did not reach statistical significance. In all disease groups GP data overestimated smoking prevalence compared with the postal questionnaire. We conclude that practices targeted their recording of smoking to those with smoking-related conditions, but achieving higher and more accurate rates of recording in those with smoking-related diseases remains a priority.