During screening of colorectal cancer by the Haemoccult test (Röhm Pharma Co.) the authors investigated the incidence of idiopathic proctocolitis in the population of the Bruntál district. The group of tested subjects comprises 16.642 of the population aged 45 to 86 years. After examination of 380 positive subjects (2.28%) twelve cases of idiopathic proctocolitis were detected. Of these twelve seven were identified previously and a new diagnosis was made in five subjects. At twelve cases were identified from a population of 16.642, the assumed prevalence in 1989 in the given age group is 72/100.000. This suggests considerable underestimation of the prevalence of idiopathic proctocolitis in current epidemiological surveys.