Transplantation of mesenchymal stem cells (MSCs) derived from synovium is a possible therapy for meniscus regeneration. We have previously reported that intraarticular injection of 5 million synovial MSCs promoted meniscal regeneration in rat meniscal defects. However, if a similar cell number per body weight were required, preparation of required human MSCs would not be practical in a clinical situation. The use of aggregates of MSCs may be one of the solutions. Here, we investigated whether the use of aggregates of synovial MSCs regenerated meniscus more effectively in a rat meniscectomized model. The total number of synovial MSCs was adjusted to 25,000 cells, and aggregates consisting of MSCs or 25,000 MSCs suspended in PBS were placed on the meniscal defects. Five million MSCs suspended in PBS were also used as another control. For the regenerated menisci, the area was larger and the histological findings were closer to that of the normal meniscus in the aggregate groups than to that in the suspension groups at 4 weeks. The effects of transplantation of aggregates were still observed at 12 weeks. Luminescence intensity remained higher at 3 weeks and thereafter in the aggregate group than in the suspension group when the same number of luciferase expressing MSCs were transplanted. We confirmed that MSCs transplanted as aggregates existed in the regenerated meniscus focally and partially. Transplantation of aggregates of synovial MSCs regenerated meniscus more effectively in a rat massive meniscal defect. (C) 2013 Elsevier Inc. All rights reserved.