Even though marijuana smoke contains carcinogens and more tar than tobacco smoke and marijuana intoxication has been implicated as a risk factor for injuries, relatively little epidemiologic evidence has identified marijuana use as a risk factor for ill health. This study is the first to examine the health effects of smoking marijuana by comparing the medical experience of "daily" marijuana smokers who never smoked tobacco (n = 452) with a demographically similar group of nonsmokers of either substance (n = 450). Marijuana smoking status was determined during multiphasic health checkups at Kaiser Permanente medical centers between July 1979 and December 1985. Medical records were reviewed for as long as 2 years after the checkups. Frequent marijuana smokers had small increased risks of outpatient visits for respiratory illnesses (relative risk [RR] = 1.19; 95% confidence interval [CI] = 1.01, 1.41), injuries (RR = 1.32; CI = 1.10, 1.57), and other types of illnesses (RR = 1.09; CI = 1.02, 1.16) compared with nonsmokers; their risk of being admitted to a hospital was elevated but not statistically significant (RR = 1.51; CI = 0.93, 2.46). Analyses were adjusted for sex, age, race, education, marital status, and alcohol consumption. Daily marijuana smoking, even in the absence of tobacco, appeared to be associated with an elevated risk of health care use for various health problems.