Reactive oxygen species cause damage to proteins, lipids and DNA. Coenzyme Q10 (CoQ10) is a compound withmitochondrial bioenergetic functions. The reduced form of CoQ10 shows antioxidant activity. In the present study,effects of CoQ10 on development of azoxymethane (AOM)-induced aberrant crypt foci (ACF) and mucin-depletedfoci (MDF) in F344 male rats were investigated. To induce ACF and MDF, 6-week old rats were given two weeklysubcutaneous injections of AOM (15 mg/kg body weight) and also received a control diet or experimental dietscontaining CoQ10 (200 or 500 ppm) for 4 weeks, starting one day before the first dose of AOM. At 10 weeks of age,all animals were sacrificed and their colons were evaluated for numbers and sizes of ACF and MDF. Administrationof 200 and 500 ppm CoQ10 resulted in reduction of ACF numbers, to 77% and 68% of the carcinogen control value,respectively. The percentages of ACF consisting of more than 4 crypts in these groups were also significantly lowerthan in the controls. Treatment with 500 ppm CoQ10 furthermore decreased the number of sialomucin-producingACF and MDF per colon to 42% and 38% of the carcinogen control value without CoQ10, respectively. Theseresults suggest that CoQ10 may be an effective chemopreventive agent against colon carcinogenesis.