In an ongoing study of the pulmonary effects of heavy, habitual marijuana smoking, detailed marijuana and tobacco smoking histories were obtained from 467 adult regular smokers of marijuana and/or tobacco. Frequency and cumulative amounts of tobacco and marijuana smoking were similar for smokers and nonsmokers of tobacco, except that pack-years and cigarettes/day at the time of the interview were both significantly less for tobacco smokers who also smoked marijuana compared those who did not. For all subjects who smoked both substances at any time, changes in tobacco and marijuana smoking amounts after commencement of regular smoking of the other substance were similar for tobacco and marijuana; the existing smoking habit decreased in approximately one third of the subjects and remained the same in slightly more than one half of the subjects. Of the dual smokers, 49% began smoking tobacco before marijuana, while 33% began smoking marijuana first; 85% of marijuana smokers who quit tobacco smoking did so after beginning regular marijuana smoking. Self-reported depth of inhalation and breath-holding time of marijuana smoke were similar for tobacco and non-tobacco smokers; smoking topography for tobacco was also comparable for smokers and non-smokers of marijuana.