The effects of early home management of acute diarrhea followed by rapid in-hospital realimentation on intestinal permeability were studied in 41 children aged 3-25 months with acute gastroenteritis (73% rotavirus). After oral rehydration, a 100 ml oral load containing 4 g of lactulose and 0.8 g of mannitol was administered, and an aliquot of urine excreted in the subsequent 5 h was analyzed with gas-liquid chromatography. The mean lactulose/mannitol recovery ratio was significantly higher than in 28 nondiarrheal controls, which was due to decreased mannitol excretion. The gastroenteritis patients who had received uninterrupted feeding in addition to adequate fluid replacement before hospitalization had a normal urinary lactulose/mannitol ratio, with a mean of 0.04, and a 95% confidence interval (CI) of [0.03, 0.07], whereas in fasted children with inadequate or adequate fluid replacement, the respective mean ratios were 0.24, 95% CI of [0.14, 0.43], and 0.14, 95% CI of [0.09, 0.20] (F = 12.63, p less than 0.001). The fasting-associated rise was caused by increased lactulose excretion. At retesting of gastroenteritis patients after 2 days of in-hospital realimentation, the lactulose/mannitol ratios did not differ significantly from the level on admission. The study indicated that fasting maintains the increased intestinal permeability associated with acute gastroenteritis whereas early feeding at home may promote reduction of permeability and hasten recovery.