Because IC varies so much in symptoms and severity, most researchers believe that it is not one, but several, diseases. In the past, cases were mainly categorized as ulcerative IC or nonulcerative IC, based on whether ulcers had formed on the bladder wall. But many researchers and clinicians have questioned the usefulness of this classification, since the vast majority of cases do not involve ulcers, and their presence or absence does not influence treatment options as much as other factors do.

Factors that influence treatment options include whether bladder capacity under anesthesia is great or small, and whether mast cells are present in the tissue of the bladder wall, which may be a sign of an allergic or autoimmune reaction. In some cases, the success or failure of a treatment helps characterize the type of IC. For example, some cases respond to changes in diet while others do not.