AIMS: To validate self-report about smoking cessation with biochemical markers of smoking activity amongst patients with ischaemic heart disease. PATIENTS AND METHODS: Outpatients at the Division of Cardiology, 75 years of age or younger, who had been Hospitalized at Sahlgrenska University Hospital in Göteborg due to an ischaemic event and who consecutively participated in a nurse-monitored routine care programme for secondary prevention, from 6 February 1997 to 5 February 1998. Data concerning smoking habits were collected through interviews. Two chemical markers, cotinine in plasma and carbon monoxide (CO) in expired air, validated self-reports concerning smoking cessation. RESULTS: 260 former smokers were validated. In the vast majority of the study population, the anamnestic information concurred with the chemical marker. However, 17 patients had chemical markers that contradicted their self-report with raised CO (n = 6) and/or raised cotinine levels (n = 13) without alternative nicotine delivery. CONCLUSION: Most patients with coronary artery disease relating information concerning cessation of smoking are truthful. A few patients, however, seem to conceal their smoking. Testing by chemical markers may be questionable for ordinary care but should, however, be included in studies concerning the association between smoking and health.