The relationship between smoking and gastric secretory capacity was studied in 201 25-40 year old healthy subjects with normal laboratory data and no gastrointestinal lesions. Basal acid output (BAO) and peak acid output (PAO) were determined in all, and basal and stimulated pepsin outputs were measured in 85 participants. The accuracy of the patients' statements was checked by urinary nicotine assay. Basal acid output and PAO were significantly higher in male smokers (n = 55) than in male non-smokers (n = 49). In women PAO in smokers (n = 38) was higher than in non-smokers (n = 59). Female smokers (n = 38) had a higher pepsin output than female non-smokers (n = 23). Eight variables were considered in relation to BAO and PAO: age, height, weight, alcohol abuse, smoking habits, duration of smoking habit, number of cigarettes per day, and the product of years of smoking multiplied by daily number of cigarettes. The daily number of cigarettes X years of smoking was most closely correlated with BAO and PAO by stepwise multiple linear regression analysis. It was concluded that smoking is related to increased gastric acid capacity.