Celiac disease is one of the most common chronic diseases encountered in the Western world with a serological prevalence of approximately 1%. Since it is so common, much comorbidity will occur either as associations or simply by chance, or as complications of the disorder. Many of the published studies purporting to establish the frequency of these occurrences have been limited by factors such as the source and number of patients considered, choice of control groups and ascertainment bias. Recent epidemiological studies have attempted to minimize these sources of error and provide more reliable information. Autoimmune diseases constitute clinically important associations, of which Type 1 diabetes mellitus and thyroid disorders are the most important. Several liver disorders, including primary biliary cirrhosis and primary sclerosing cholangitis, are also associated. The frequency of malignant complications of celiac disease is much lower than earlier studies have indicated, with lymphoma increased by approximately fivefold and the absolute number of tumors is small. The increase in fracture risk in celiac disease is only modest. Although neurological and psychiatric conditions affect celiac patients, no disorder specifically associated with celiac disease has been identified. Reproductive problems have been overexaggerated. It is important that these co-morbidities are recognized because if not, symptoms will be falsely attributed to deliberate or inadvertent ingestion of gluten, rather than prompt a search for a second diagnosis. Furthermore, in a patient with an established diagnosis that is considered falsely to account for the whole clinical picture, celiac disease is likely to remain undetected.