In this paper, we evaluate the performance of sonoelastography for prostate cancer detection. Ultrasound (US) B-mode and sonoelastographic volumes were acquired from five prostate glands ex vivo. Additionally, one more gland was imaged in vivo using a transrectal US probe. Semi-automatic algorithms were used to segment the surface of the gland from the B-mode volume and the tumors from sonoelastographic data. To assess the detection performance, three dimensional (3D) sonoelastographic findings were compared in size and position to 3D histological data. Sonoelastography detected seven out of nine cancers in the ex vivo prostate glands and two out of three malignant masses in the in vivo experiment. Overall, 3D sonoelastography has shown potential for prostate cancer detection albeit based on limited data.