To determine whether diarrhea increases fecal fat excretion, fecal fat output was measured during three consecutive days in normal subjects and in normal subjects in whom diarrhea was purposefully induced. Mean fecal fat output in normal subjects was 3.4 g/day; the upper limit of normal was 6.4 g/day. Twenty-one percent of subjects with mild to moderate diarrhea (fecal weight 200-800 g/day) and 58% of subjects with severe diarrhea (fecal weight greater than 800 g/day) had fecal fat excretion above the upper limit of normal, with values as high as 13.6 g/day. It is concluded that (a) diarrhea itself can induce mild secondary steatorrhea, and (b) when the quantitative fecal fat test is used in patients with diarrhea, mild abnormalities (up to 14 g/day) are not specific for a primary defect in fat digestion or absorption, i.e., they may represent false-positive results.