The geographic and temporal variations in mortality from Crohn's disease and ulcerative colitis were investigated. The validity of mortality data as indicators of morbidity was tested by comparing the death rates and incidences among different countries. Death rates from Crohn's disease and ulcerative colitis were high in England, Germany, and the Scandinavian countries, and low in the Mediterranean countries. There was a significant correlation between the incidence and mortality of both diseases among different countries. In addition, the incidence and mortality of Crohn's disease were correlated with those of ulcerative colitis. In countries with a low mortality rate from Crohn's disease, the death rates in men tended to be higher than those in women. In contrast, countries with high death rates from Crohn's disease showed female predominance. No such relationship existed for ulcerative colitis. The overall change in mortality rates during the last 20 to 30 years was characterized by a rise of Crohn's disease and a marked fall of ulcerative colitis. In countries with a high mortality rate from Crohn's disease, the death rates started to fall in recent times. The significant correlations between incidence and mortality show that the death rates from both diseases represent reliable indicators of the morbidity and that the severity of the two diseases is similar in different countries. The marked temporal and geographic variations in both incidence and mortality suggest that environmental factors play an important role in the etiology of both diseases.