In 1985, a histopathological examination was conducted in the ileum and a parasitological examination in the ileum and rectum of 345 emergency slaughtered calves. The pathogenic role of cryptosporidia was compared in two groups of calves: group A contained 184 calves (53.3%) with scours and enteritis, group B contained 161 calves (46.7%) with other types of disease. In both groups the calves were divided into four age categories: 7 to 10 days (7.8%), 11 to 14 days (29.3%), 15 to 21 days (53.0%), 22 days and older (9.9%). During the parallel examination by both methods, cryptosporidia were detected in 124 cases, i.e. at the total invasion extensity of 35.93%; in group A the cryptosporidia were found in 76 cases (22.02%) and in the group B in 48 cases (13.91%). As indicated by the results, cryptosporidiosis is a disease of polyfactorial origin and cryptosporidia must be taken into account as one of the enteropathogenic factors in the etiology of scours. It is confirmed by the positive findings of cryptosporidia in the calves of group B (29.81%) and the negative finding in group A (31.3%) that no clear relationship was demonstrated to exist between the positive findings of these protozoans and the clinical symptoms of the diseases and enteritis. The pathogenesis of cryptosporidiosis includes significant regressive changes in the microvillous layer and enterocytes, which were a common finding at heavy invasions. The total positivity of the findings of cryptosporidia in the ileum was significantly higher (by 14.8%) during the histopathological examination, as compared with the parasitological examination. Out of the total positive findings, 39 cases (31.5%) were demonstrated histopathologically. During the parasitological examination, the positivity of the findings of cryptosporidia was higher by 4.3% in the rectum than in the ileum. When there was conducted a parallel parasitological examination of the ileum and rectum, the total positivity was higher by 7.5% than at the histopathological examination in the ileum. The highest invasion extensity was found at the age of 7 to 10 days in both groups of calves. The highest invasion intensity was found at the age of 11 to 21 days in both groups. The age until the 21st day when the parasitosis is intensively spread is considered as the most dangerous age of the calves from the epizootological point of view. Cryptosporidia occurred all the year round, no characteristic seasonal patterns were observed. The total average invasion extensity of the 24 farms from where the calves came was 42.3%, its range being from 0 to 85.7%.