Piglets from five litters were doses orally with cryptosporidium originally derived from diarrhoeic calves. The piglets were either nursed by the sow, artificially reared after sucking colostrum, or weaned on to creep feed. Colostrum-fed, artificially reared piglets obtained from two litters and exposed in the first week of life developed clinical signs of inappetence, vomiting and diarrhoea and shed oocysts in the faeces. Histologically the parasite was observed throughout the small and large intestine attached to epithelial cell surfaces and its presence was associated with extensive mucosal damage, particularly in the posterior small intestine, stunting and fusion of villi, immaturity of villous epithelial cells and oedema with increased cellularity of the lamina propria. Piglets from two other litters, both sucking and colostrum-fed artificially reared, exhibited similar but milder clinical signs. Histological lesions were less severe and cryptosporidium infection less extensive. When weaned piglets were exposed they remained clinically healthy although histologically there was evidence of cryptosporidium attachment in the small intestine and minor mucosal damage. There appears to be a good correlation between the extent of intestinal infection, the degree of mucosal damage and the severity of clinical disease induced by cryptosporidium in piglets.