The effect of dietary yeast on the activity of stable Crohn's disease was assessed in 19 patients. During the 1st month patients continued their usual diet (base-line period), but during the next 2 months dietary yeast was excluded except that during I month patients took baker's yeast capsules while for the other month they took placebo capsules. The patients' mean Pettit Crohn's disease activity index (CDAI) while taking baker's yeast (mean, 107.9; SE, 6.1) was significantly greater than during yeast exclusion (mean, 102.1; SE, 5.7; p < 0.05). The mean of each patient's maximum CDAI during yeast exclusion (mean, 107.1; SE, 5.7) was significantly lower than those during the base-line (mean, 115.2; SE, 6.1; p < 0.05) and baker's yeast inclusion periods (mean, 113.9; SE, 6.7; p < 0.05). Patients with elevated yeast antibodies tended to develop a higher CDAI while receiving baker's yeast (13 of 15). These results suggest that dietary yeast may affect the activity of Crohn's disease.