Mycobacterium avium subspecies paratuberculosis (MAP) has been suspected of involvement in Crohn's disease (CD). We investigated this potential association by testing whole blood from CD patients and healthy controls for the presence of MAP by culture and molecular methods. In addition, each blood sample was analyzed for polymorphisms in the NOD2/CARD15 gene previously associated with CD.Four 4-mL K(2)-EDTA tubes of whole blood were drawn from each subject (n = 260, 130 CD patients and 130 healthy controls). Two tubes of blood were cultured for MAP by the following methods: Mycobacterial Growth Indicator Tube, Herrold's Egg Yolk Agar, BACTEC 460, and Hungate. The remaining 2 tubes of blood were tested for MAP DNA and polymorphisms in the NOD2/CARD15 gene by polymerase chain reaction (PCR).One healthy control patient was positive for MAP via PCR; however, no viable MAP was cultured from this individual. All blood cultures were negative for MAP. One CD patient's blood was culture-positive for M. tuberculosis complex. CD patients exhibited a higher rate of polymorphism in the NOD2/CARD15 gene than healthy control patients.In this study MAP was not recovered from the blood of CD patients or healthy controls. However, CD patients showed higher mutation rates in the NOD2/CARD15 gene, compared with healthy controls, supporting the findings of other investigators. No correlation between these polymorphisms and MAP bacteremia in CD patients could be identified in this study.