To evaluate genotoxicity of anabolic androgenic steroids (AAS) in male bodybuilders by a micronucleus assay in buccal mucosa cells. 11 male bodybuilders volunteered to participate in this study and two groups were formed: group 1 (n = 6), without AAS consumption and group 2 (n = 5), with AAS consumption. A sample of buccal epithelium was taken from each participant once a week for 6 weeks. Samples were fixed, stained and analysed by a light microscope, and 2000 cells were counted from each slide. Results are expressed as micronucleated cells (MNC) per 1000 cells and were analysed by the Mann-Whitney U test and Wilcoxon's test. A marked increased in MNC was seen in bodybuilders with AAS consumption compared with those without AAS consumption (mean (SD) 4.1 (2.4) MNC/1000 cells vs 0.4 (0.4) MNC/1000 cells, respectively; p