Much less is known of the effects of stopping smoking than of continuing to smoke, and many of the studies on smoking cessation are concerned with long-term effects rather than effects within 48 hr. Studies concerned with this period are required, especially in terms of postoperative respiratory morbidity, before an authoritative assessment can be made of the benefits and risks of stopping smoking in the short period before operation. Present studies are convincing that great benefit will accrue in the cardiovascular system, mainly from carbon monoxide and nicotine elimination, after 12-24 h. A few days may greatly improve ciliary beating and 1-2 weeks provide a significant reduction in sputum volume. However, a minimum period of 4-6 weeks would seem appropriate to greatly influence postoperative respiratory morbidity, although the statement that "one needs 4-6 weeks to influence postoperative respiratory morbidity" must not be misapplied and become "there is no point in giving up smoking unless it is 4-6 weeks prior to operation." There are no proven disadvantages to the respiratory system from stopping smoking in the short term, and it seems unwise to sacrifice proven advantages for a theoretic consideration that sputum may become "stickier" and more difficult to clear. Less is known with regard to the time course of offset of smoking effects on drug metabolism and the immune system, although 6-8 weeks would be expected to produce some benefit.(ABSTRACT TRUNCATED AT 250 WORDS)