The present study evaluated the intraindividual changes of serum lipids, blood pressure, adrenergic stimulation, and platelet reactivity in chronic smokers undergoing controlled smoking cessation assisted by nicotine chewing gum. One hundred twenty-one healthy smokers, who had smoked at least 20 cigarettes a day for at least 5 years, were included in the study. Serum lipids, blood pressure, catecholamine concentrations in serum and urine, platelet volume and platelet function in vitro were assessed during an initial phase of continued smoking, after stopping smoking during nicotine gum chewing, and after abstaining from both smoking and gum chewing. After a median of 11 weeks of chewing nicotine gum, 52 persons (43%) remained abstinent from smoking and chewing for at least 12 weeks. In these successful study participants, low-density lipoprotein (LDL)-cholesterol levels decreased significantly by a mean of 7.5 mg/dL (5.6%). High-density lipoprotein (HDL)-cholesterol increased by 5 to 6 mg/dL during the first weeks after ceasing to smoke, but levels at the end of study were only 2.1 mg/dL (3.4%) higher than baseline (nonsignificant difference). Triglyceride levels did not change during smoking cessation. While systolic blood pressure remained unchanged, diastolic blood pressure increased significantly by a mean of 3.5 mm Hg. For LDL-cholesterol, HDL-cholesterol, and systolic and diastolic blood pressure, there was an inverse correlation between the respective baseline values and the observed changes during the cessation of smoking. The urinary excretion of adrenaline and noradrenaline decreased upon quitting. The volume of platelets became significantly less during smoking cessation, but aggregation induced by adenosine diphosphate (ADP) and collagen and thromboxane synthesis in vitro remained uninfluenced. The platelet cyclic adenosine monophosphate (cAMP) response to stimulation of adenylate cyclase with prostaglandin E1 showed marked enhancement upon quitting. The changes observed intraindividually during smoking cessation largely correspond to the differences between smokers and nonsmokers described in epidemiologic studies. For serum lipids and blood pressure, a significant dependence of the changes on the respective values at baseline may indicate greater benefit for persons at higher risk. Decreased platelet volume and increased susceptibility of platelets to antiaggregatory prostaglandin E1 indicate a favorable influence of quitting on platelet reactivity.