Ultrastructural investigation of detrusor muscle cells in biopsies from 13 patients (8 interstitial cystitis (IC) and 5 controls) was performed. In all IC-patients and in one control a varying number of smooth muscle cells revealed a characteristic oak leaf pattern with protrusions of the sarcolemma. The alterations may express degeneration of smooth muscle cells. Although most probably non-specific yet highly characteristic, the ultrastructural detrusor changes may be important in diagnosing IC.