AIMS: To define the pathology of painful bladder syndrome using a morphometric method. METHODS: Bladder biopsy specimens from 31 patients with painful bladder syndrome and 11 controls were stained and examined at x260 magnification with the aid of a 100 square counting grid. Random counts of the different tissues and inflammatory components were made to ascertain whether constant differences occurred that could be used to define the pathology of this uncommon condition. RESULTS: In the lamina propria of painful bladder syndrome specimens, a significant increase was seen in the concentration of lymphocytes, T cells, and blood vessels; a decrease was seen in the number of fibroblasts, and no change was seen in the number of mast cells and macrophages. B cells were sporadic. The basement membrane in these specimens showed significant discontinuity and there was increased collagen deposition in the underlying muscle when compared with controls. CONCLUSION: Painful bladder syndrome exhibits constant histological features that may be used to aid diagnosis in this uncommon condition. Simple numerical cell/tissue measurement of this kind is also useful when treatment trials are considered, because objective statistical analysis (pretreatment and post-treatment) is possible without the need for expensive and complicated equipment.