Chronic abacterial cystitis is clinically and pathoanatomically an ill-defined condition, presenting with a variety of urologic symptoms and often nonspecific histology. A retrospective histologic analysis of bladder biopsies from 101 patients with chronic abacterial cystitis revealed degenerative changes in the detrusor muscle cells (detrusor myopathy) in 25 of them. The changes were often very severe, and even fatty replacement of muscle tissue was seen. Retrospective analysis of the symptoms, urodynamics and cystoscopic findings in these patients showed that none had haematuria, but five (all women) had had urinary retention requiring catheterization. Significant residual urine was found in nine patients, and the cystometrograms showed a shift to the right. No patient had infravesical obstruction. At cystoscopy less than half of the patients had petechial bleeding after bladder distension. The aetiology and pathogenesis of the degenerative changes are unknown, despite theoretic speculation. The authors conclude that careful histologic examination can identify different clinical types of chronic abacterial cystitis. Such studies may assist future research into the nature of the problem and also the search for more rational therapy.