One of the reasons for the paucity of tabagism exposure data on the consequences of smoking is the difficulty in obtaining urine samples and the fact that the optimal storage conditions remains undetermined. The authors therefore assessed the influence of storage on urinary nicotine and cotinine levels both at room temperature and after freezing. The variations observed were not statistically significant for up to 30 hours at room temperature or for up to 8 days at -25 degrees C. They then studied the excretion of cotinine and nicotine in overnight and 24-h urine specimens collected from 90 non-smokers exposed to tobacco smoke and 40 smokers. The correlation between overnight and 24-h excretion was excellent in the case of cotinine (r = 0.89) and poor for nicotine (r = 0.18), probably because of their respective half-lives. Lastly, the usefulness of referring the urinary cotinine to the urinary creatinine was questioned. The authors conclude that valuable studies should be based on overnight urines samples stored at room temperature for up to 30 hours and then frozen at -25 degrees C until quantification of cotinine expressed in microgram/fraction.