Monocytopoietic proliferation activity was determined in 8 patients during severe attacks of Crohn's disease and in 6 patients with ulcerative colitis. Similar results were obtained in both groups of patients. A moderate but significant hyperproliferation of monocytopoiesis was found to be present in about half of the patients, and with some of the remainder of cases, part of the criteria for hyperproliferation were also fulfilled. This indicates that Crohn's disease as well as ulcerative colitis are frequently associated with moderate overproduction of monocytes which may be assumed to be induced by macrophage demand of the affected tissues. In comparison with other diseases involving inflammations, the monocytopoietic hyperproliferation was moderate. Therefore, the inflammation in Crohn's disease and ulcerative colitis seems to be characterized by a relatively low macrophage turnover induced by pathogenetic mechanisms of moderate macrophage toxicity.