To assess the reliability of the erythrocytic sedimentation rate (ESR) as a measure of clinical activity in inflammatory bowel disease, we analyzed the correlations of ESR with a global assessment of clinical activity in 77 patients with varying extents of Crohn's disease and ulcerative colitis. Analysis of all 141 ESR determinations in all 77 patients showed a highly significant correlation between mean ESR and clinical activity score (r = 0.54, p less than 0.001). Analysis of 133 ESR determinations in these 77 patients when their disease activity was either mild, moderate, or severe showed some significant differences among certain disease categories. The highest mean ESRs were in patients with the most extensive colon involvement (Crohn's colitis 40.7 +/- 3.3, universal ulcerative colitis 31.0 +/- 3.9), whereas the lowest mean ESRs were in patients with the most limited disease (ulcerative proctitis and proctosigmoiditis 19.2 +/- 2.1). The rate of increase in ESR with progressively increasing clinical activity from mild to moderate was the same in all disease categories, with the exception of Crohn's disease limited to the small bowel (ileitis or jejunoileitis), in which the ESR was relatively unchanged in a small sample of patients. By the time clinical activity became severe, however, patients in all disease categories manifested similarly high ESRs, with the exception of ulcerative proctitis in which the ESR remained low in the single patient tested.(ABSTRACT TRUNCATED AT 250 WORDS)