Induratio penis plastica (Peyronie's disease) is a chronic fibrotic process involving the penis. Proper treatment of the disease requires assessment of the degree of inflammation preceding or accompanying the fibrous Peyronie's plaques. Owing to its high tissue contrast and its multiplanar capability, MR imaging offers excellent visualization of penile anatomy. To determine the usefulness of MR imaging in the diagnosis and staging of Peyronie's disease, we used MR imaging with a surface coil to examine 28 consecutive patients with clinical evidence of the disease. Eighteen patients had contrast-enhanced MR imaging with gadopentetate dimeglumine. In seven patients who subsequently had surgery or biopsy, MR findings were correlated with histopathologic findings. On unenhanced images, fibrous plaques were shown in 20 patients. Enhanced MR images showed focal contrast enhancement around or within the plaques in seven patients. Images in three patients with plaques showed no enhancement. Images in five patients showed focal areas of contrast enhancement without evidence of plaques. Histologic studies demonstrated that the degree of contrast enhancement correlated with the extent of inflammatory cell infiltration. In two patients with unenhancing plaques on MR, histology confirmed the absence of inflammation. Our results suggest that MR imaging not only depicts the localization and extent of fibrous plaques in patients with Peyronie's disease but also reveals the presence of inflammation. This makes MR imaging the technique of choice for planning therapy and for evaluating the response to conservative treatment.