The author investigated in 130 anatomical specimens the topographical relations of the sciatic nerve and the musculus piriformis in order to explain the clinical syndrome of the m. piriformis. The author found that in 6.15% of cases the nervus peroneus communis passes between the tendinous parts of the m. piriformis, and he considers this variation of practical significance for the development of the ‘piriformis syndrome’. In unward rotation of the thigh, the m. piriformis is extended and the tendons of the divided muscle are tightly pressed together thus pinching the nerve between them. Pinching of the nerve causes the characteristic sciatic pain. In such a case, the patient can be relieved by cutting one of the tendons.