Purpose: We tested whether the types of inflammatory cells seen on bladder biopsies were associated with other clinical features and urinary markers of interstitial cystitis. Materials and Methods: Bladder biopsies from 30 interstitial cystitis patients were evaluated by immunohistochemical staining for T cells, B cells, macrophages and human leukocyte antigen-DR positive cells. These findings were tested for associations with clinical features and urinary markers of interstitial cystitis using alpha = 0.01 because multiple tests were performed. Results: Overall severity of inflammation was significantly associated with age at symptom onset, symptom relief after bladder distention and urinary interleukin-6 levels. Patients with severe inflammation had trends toward smaller bladder capacity under anesthesia, increased bladder vascularity and mucosal cracks, lower urinary MUC-1 glycoprotein levels and absence of bloating as a symptom. B cell staining was significantly associated with severe inflammation, symptom relief after distention and absence of bloating as a symptom. T cell staining was significantly associated with severe inflammation and age at symptom onset. Human leukocyte antigen-DR staining had trends with symptoms, including presence of bloating, constant urge to void and absence of burning. Macrophage staining did not associate with any features tested at the alpha = 0.05 level. Conclusions: Interstitial cystitis patients with severe inflammation have different age, treatment response and urinary marker levels than those with mild inflammation. These findings suggest that the 2 patient groups have different underlying pathophysiologies. The significant associations for T and B cell staining were similar to those for overall inflammation.