Sera from patients with Crohn's disease (CD) and ulcerative colitis (UC) have been evaluated for antibodies reactive with Saccharomyces cerevisiae (anti-Sacc antibodies) using an enzyme-linked immunoassay (ELISA). IgG anti-Sacc antibodies were detected in 63% (25/40) of CD patients, compared with 15% (4/27) of UC patients (p less than 0.001) and 8% (5/60) healthy adult controls (p less than 0.001). Furthermore, the prevalence of detectable IgG anti-Sacc antibodies in adult patients with coeliac disease, dermatitis herpetiformis, irritable bowel syndrome or atopic eczema was not significantly different to controls. In comparison, the prevalence of detectable IgG anti-Escherichia coli antibodies was not significantly different between CD (75%) or UC (79%) patients. More particularly, elevated levels of serum IgA anti-Sacc antibodies were detected in 17/40 CD patients, but in none of the 27 UC patients. These data confirm that serum antibodies reactive with S. cerevisiae are strongly associated with CD and further show that serum IgA anti-Sacc antibodies may be specific for this disorder.