We report a prospective series of 33 unicompartmental knee arthroplasties (UKAs) operated for a spontaneous osteonecrosis of the knee (SPONK) compared with 35 UKAs operated for osteoarthritis (OA). The mean follow-up was 5 years. Preoperative functional score in the SPONK group was significantly lower than that in the OA group. The results were comparable in terms of pain, knee score and function. At the last follow-up, the survival rate was 92.8% for the SPONK group and 95.4% for the OA group. We found a higher rate of radiolucencies in the SPONK group, however, without any clinical symptoms. The UKA is a good option in the treatment of SPONK.