The metabolism of erythritol was assessed in six normal volunteers by measuring the amount of 13CO2 excretion and H2 excretion in breath, and erythritol in urine after intake of 25 g 13C-labelled erythritol. The results were compared with the same variables obtained after intake of 25 g 13C-labelled glucose and 13C-labelled lactitol. In addition, the H2 production by faecal flora supplemented with small amounts of erythritol, glucose and lactitol was measured in vitro, as an index of bacterial metabolism of non-absorbed substrate. In contrast to the results obtained after intake of glucose and lactitol, no increase in breath 13CO2 and H2 was observed after intake of erythritol, and erythritol was nearly completely recovered in urine. The in vitro experiments showed that no H2 was formed by faecal flora from erythritol as compared with glucose and lactitol. It is concluded that erythritol is a substrate that is readily absorbed, and undergoes no metabolism by the host. If part of it escapes absorption, it is not metabolized by faecal flora.