As part of a larger study into the effects of polyphenols derived from chocolate on bowel health we have compared the effects of consumption of chocolate containing either 200 mg of flavanols and related procyanidins or a similar chocolate containing less than 10 mg of polyphenols on fecal free radical production and antioxidant activity in 18 volunteers. In a double-blind crossover trail volunteers consumed chocolate for two 4-wk periods separated by a 4-wk washout period. During the time the volunteers consumed the chocolate they also consumed a low-polyphenol diet. Free radical production in the fecal water was lowered from 122 +/- 10 micromol/l/h to 94 +/- 9 micromol/l/h (P = 0.009) when the high procyanidin chocolate diet was consumed and from 117 +/- 14 micromol/l/h to 86 +/- 12 micromol/l/h when the low procyanidin chocolate was consumed (P = 0.014). Fecal water antioxidant capacity measured by either the Trolox equivalent antioxidant capacity or ferric reducing ability of plasma procedure was not significantly affected. Consumption of either chocolate reduced the production of free radicals in fecal water. This suggests that some component of the chocolate other than the flavanols and related procyanidins may have been effective.