Erythema nodosum (EN) has been reported in patients with ulcerative colitis (UC) since 1909. The reported incidence varies from 0.9 p. 100 to 18.9 p. 100. A wide range of incidence of EN has also been found in Crohn's disease (CD), with figures varying from 0.7 p. 100 to 8 p. 100 (fig. 1). Different criteria for the diagnosis of EN, UC and CD probably account for the scattering of values. A review of 324 cases of inflammatory bowel diseases revealed 21 cases with one or more episodes of EN: 6 out of 195 cases of UC (3.1 p. 100) and 15 out of 129 cases of CD (11.6 p. 100). The incidence was higher in females (5.7 p. 100 in UC, 17.5 p. 100 in CD) than in males (0 in UC, 6.9 p. 100 in CD). At the time of the eruption, patients with CD were younger (mean 24.3 years) then those with UC (mean 37.5 years). The interval between the onset of the intestinal symptoms and the nodular eruption was shorter in patients with CD (2.6 years) than in those with UC (8.2 years). The inflammatory bowel disease (whether UC or CD) started earlier in patients with EN than in EN-free patients. EN antedated the onset of intestinal manifestations in one patient with UC. In the remaining 20 patients the intestinal symptoms came first. EN was often recurrent, and the 21 patients suffered 32 episodes at varying intervals. One patient had 4 and another 3 episodes; six patients had 2 and 13 had one single episode. Three patients with UC had 2 episodes and 5 patients with CD had 2 or more episodes. The morphology and distribution of the lesions was fairly classical, except in 2 patients who had only 1 and 2 nodules respectively on one leg. The eruption subsided in all cases within 2 to 5 weeks, and no atrophy, suppuration or ulceration was observed. Most episodes of EN occurred during active phases of the intestinal disease. This was not so, however, in 2 cases in which the eruption followed an acute streptococcal throat infection. EN was more often found in total UC than in the less extensive distal and rectal types (Table I). It was also more frequent in the ileocolic form than in the exclusively ileal and colonic forms (Table II).(ABSTRACT TRUNCATED AT 400 WORDS)