Acute diarrhea caused by Cryptosporidium-protozoan is rarely diagnosed in Finland. The infection is usually self-limited and does not require antimicrobial treatment. Cryptosporidiosis, like other intestinal parasite infections, is mostly associated with travelling, but may also cause large waterborne epidemics. Contact with infected calves may be a source of cryptosporidiosis also in Finland. Cryptosporidiosis should be considered in patients suffering from severe or long-lasting watery diarrhea. We describe three cases of cryptosporidiosis, originating from infected calves. These cases show that verification of the etiology of human cryptosporidiosis associated with calves may be difficult and demands collaboration of clinicians, laboratories and veterinarians.