OBJECTIVE: Anti-Saccharomyces cerevisiae mannan antibodies (ASCA) are associated with Crohn’s disease. The aim of this study was to determine the prevalence of ASCA in families in which at least two members were affected with Crohn’s disease. METHODS: A total of 20 families including two (n=15) or more (n=5) patients with Crohn’s disease were tested for ASCA with use of an ELISA method. Overall, 51 affected members, 66 healthy first degree relatives, and 163 healthy control subjects were studied. RESULTS: ASCA were detected in 35 of 51 (69%) patients with Crohn’s disease and in 13 of 66 (20%) healthy relatives versus one of 163 healthy control subjects (p < 0.0001 and p < 0.001). ASCA-positive relatives were distributed in 12 of 20 families. ASCA were present in eight healthy parents and four healthy siblings. The prevalence of ASCA in relatives did not depend on the ASCA status of affected members. CONCLUSION: ASCA in 20% of healthy first degree relatives of patients with Crohn’s disease suggest that these antibodies might be a subclinical marker for Crohn’s disease in families. Whether ASCA reflect environmental or genetic factors or a combination of both is unknown.