OBJECTIVE Cigarette smoking causes cardiovascular (CV) disease, but the relative roles of nicotine and other components of tobacco smoke remain unclear. We investigated the effect of stopping smoking, by using nicotine replacement therapy (NRT), on haemorheology parameters. METHODS Open, parallel-group trial (intervention group and control smokers). METHODS Clinic within university department of pharmacology. METHODS One hundred and ninety-seven males, aged 25-45 years, smoking >20 cigarettes per day. METHODS One hundred and sixty-four subjects were instructed to stop smoking and received NRT for 12 weeks and 33 acted as controls. After 12 weeks, NRT was discontinued, and all subjects were followed-up at 26 weeks. At the end of the study, the NRT group was divided into abstainers (self-reported, verified by exhaled carbon monoxide <10 ppm) and relapsers, who were unable to remain abstinent. METHODS Plasma viscosity, fibrinogen, erythrocyte deformability, reactive capillary blood flow, transcutaneous partial oxygen tension (tcpO2) and haematocrit, assessed at 4, 8, 12, and 26 weeks. Results. After 6 months, plasma fibrinogen (9.95 vs. 8.24 micromol x L(-1) at baseline; P < 0.003), reactive capillary flow (t-pmax: 9.3 vs. 11.2 s at baseline; P < 0.005), and tcpO2 (50.4 vs. 34.9 mmHg at baseline; P < 0.0001) were significantly improved in abstainers, but changes in plasma viscosity and erythrocyte deformability were inconclusive. Other CV risk factors, such as haematocrit and white blood cell count, decreased to a greater extent in abstainers than in relapsers. Expired carbon monoxide concentrations reflected the changes in smoking and decreased in abstainers from 30.4 ppm at baseline to 4.2 ppm; P < 0.0001). Conclusions. Smoking cessation improved CV parameters, and use of nicotine medications did not negate these improvements.