Tobacco smokers are more likely to use marijuana than those who do not smoke tobacco. Little is known about how marijuana use affects the probability of tobacco smoking cessation. This analysis was based on 431 adults less than 45 years of age who reported recent tobacco smoking in the 1981 baseline interview in the household-based Baltimore Epidemiologic Catchment Area study and were re-interviewed 13 years later. At baseline, 41% of the tobacco smokers reported ever use of marijuana, 27% reported use of marijuana in the previous 30 days, and 9% reported daily use of marijuana for 2 weeks or more in the last 30 days. Marijuana users in the past 30 days at baseline were more likely than nonusers to still be using tobacco at follow-up after adjusting for race, educational level and marital status (OR = 1.94, 95% CI = 1.03, 3.63). Daily use of marijuana at baseline was even more strongly related to continued tobacco smoking 13 years later. Difficulty in tobacco cessation might be considered one of the most important adverse effects of marijuana use. Clinicians working with patients who are trying to stop tobacco smoking may be aided by routinely assessing marijuana use history, particularly with the recent increase in co-smoking of marijuana and tobacco.