The modifying effects of dietary feeding of powdered broccoli sprout (PBS), which contained a high concentration of sulforaphane, on the development of azoxymethane (AOM)-induced colonic aberrant crypt foci (ACF) were investigated in male F344 rats. We also determined the influence of PBS on proliferating cell nuclear antigen (PCNA) index in “normal-appearing” crypts from rats treated with AOM and/or PBS, and activities of detoxifying enzymes of cytochrome P450 (CYP) and glutathione S-transferase (GST) in the liver. Rats were given two weekly subcutaneous injections of AOM (20 mg/kg body wt) to produce ACF. They also received experimental diets containing PBS at a dose of 20 or 100 ppm for 4 weeks, starting 1 week before the first dose of AOM. AOM exposure induced a substantial number of ACF (106 ± 10) at the end of the study (week 4). Dietary administration of PBS caused significant reduction in the frequency of ACF: 20 ppm PBS, 56 ± 11 (47% reduction, P<0.001) and 100 ppm PBS, 64 ± 23 (40% reduction, P<0.001). Immunohistochemically, PBS administration at both doses significantly lowered the PCNA index in “normal-appearing” crypts. In addition, feeding with PBS significantly reduced the activities of CYP 2B and CYP 3A, and slightly increased GST activity in the liver. These findings suggest a possible chemopreventive ability of PBS through alterations to the activities of cell proliferation in cryptal cells in the colon and activities of drug metabolizing enzymes in the liver.