Urinary phenol is a product of dietary tyrosine metabolism generated in the gut by bacteria. In our previous study we found in 42 patients with Crohn's disease a significantly higher level of urinary phenol in terminal ileitis and ileal resection when compared with Crohn's segmental colitis. Urinary phenol is believed to be extensively influenced by diet. For this purpose our present paper investigated urinary phenol in Crohn's disease before and after two weeks of total parenteral nutrition. Spectrophotometric analysis of urinary phenol was carried out in 10 patients (7 males, 3 females). They ranged from 24 to 40 (average 34.9) years of age. The patients received on an average 1.45 g of amino acids/kg/day (no tyrosine; 0.022 g of L-phenyl-alanine/kg/day), 25-30 kcal/kg/day (105-125 kJ/kg/day) in non-protein sources of energy, and fat-emulsions (80 g/day) twice a week. There was a significant decrease in the urinary phenol after two weeks of total parenteral nutrition (median 79.5, interquartile range 41.5 to 288.5 mumols/24 hours vs. median 37.5, interquartile range 0-93.5 mumols/24 hours), p = 0.032 using the paired t-test. In two patients with severe involvement of the gut, urinary phenol, though decreased, remained at higher levels (above 300 mumols/24 hours). A significant decrease of the urinary phenol can be explained in particular by exclusion of dietary proteins, although clinical and nutritional improvement was observed, too. However, persisting higher levels of urinary phenol in some patients with serious involvement of the gut may reflect the severity of Crohn's disease.