The digestion of four sources of resistant starch (RS) has been studied in twelve healthy volunteers who ate controlled diets for 15 d periods. RS from potato, banana, wheat and maize (17−30 g/d) was compared with a starch-free diet, a diet containing wheat starch that was fully digested in the small intestine, and with 18·4 g NSP from bran/d. RS increased stool wet weight by 1·6 g/d per g RS fed for potato, 1·7 for banana, 2·5 for wheat and 2·7 for maize, but this was significantly less than bran NSP at 4·9 g/g. RS was extensively digested in twenty-seven of thirtyfour diet periods but five subjects were unable to break down one or two of the RS sources. Faecal N and energy excretion were increased. RS decreased NSP breakdown and RS2 (resistant starch granules) tended to prolong transit time. All forms of RS increased faecal total short-chain fatty acid excretion. RS2 (from potato and banana) gave greater proportions of acetate in faeces, and RS3 (retrograded starch from wheat and maize) more propionate. We have concluded that RS2 and RS3 are broken down in the human gut, probably in the colon although in 26% of cases this breakdown was impaired. RS exerts mild laxative properties, predominantly through stimulation of biomass excretion but also through some sparing of NSP breakdown.