It is recognized that interstitial cystitis (IC) is often associated with a number of diseases such as allergies, irritable bowel syndrome, fibromyalgia, inflammatory bowel disease (Crohn's disease and ulcerative colitis), systemic lupus erythematosus, and Sjögren's syndrome. The prevalence of allergies in IC is reported to be between 40 and 80% of patients. The background of the association is not known and does not seem to be the result of a generalized allergic constitution. Some report that treatment of allergy sometimes has a beneficial effect on bladder symptoms. An increased expression of certain growth factors (PD-ECGF, FGF, and VEGF) has been found in the bladder of IC patients. In addition, the expression of CD44 was significantly higher in ulcer type IC than in non-ulcer type. In general, these growth factors are soluble and diffusible under normal conditions but proteoglycans such as CD44 could bind to these growth factors. This may promote the accumulation of growth factors at the sites of inflammation. CD44 expression in ulcer type IC could explain the prolonged and stronger expression of GAG-binding growth factors that promote inflammation. In the Rotterdam study, a strong association between IC and Sjögren's syndrome is recognized. In this study, 28% of IC patients have definite or probable Sjögren's syndrome. This association is much stronger than is generally recognized. This could be due to difficulties in diagnosing Sjögren's syndrome since a diagnosis usually requires a high index of suspicion.