The effects of cannabis/methaqualone/tobacco smoking on the epithelial cells of the tongue, buccal mucosa and floor of the mouth were examined. Oral mucosal smears for detection of cellular changes were taken from 4 sites in 16 patients. The tongue blade scraping technique was used. The sites sampled included the buccal mucosa (left and right sides), the posterior dorsum of the tongue and the anterior floor of the mouth. Tobacco smoking and non-smoking controls were also examined. The only significant difference between cannabis users and controls was the greater prevalence of bacterial cells in the smears taken from cannabis users. However, there were also greater numbers of degenerate and atypical squamous cells in cannabis smokers than in cigarette-smoking and non-smoking controls. Epithelial cells in smears taken from cannabis users and tobacco-smoking controls also showed koilocytic changes, which were not seen in smears taken from non-smoking controls. Koilocytosis is indicative of human papilloma virus infection, although no apparent lesions were seen in the patients from whom smears had been taken. It would appear that there is a greater tendency towards damaged and immature surface epithelial cells in cannabis smokers.