PURPOSE: To describe modes of transarticular invasion, with reference to the size and location of a tumor, the anatomic characteristics of invaded cartilage, and the existence of ankylosis in SI joint. MATERIALS AND METHODS: Eleven histologically confirmed malignant pelvic bone tumors involving transarticular invasion of sacroiliac joints, were retrospectively analysed. Transarticular invasion of a joint was defined as involvement of its opposing bones. The anatomic site and size of the tumors were analysed, and invaded sacroiliac joint was divided into upper, middle and lower parts on the basis of the anatomic characteristics of the intervening cartilage: synovial hyaline or fibrous ligamentous. The existence of ankylosis was determined, and transarticular invasion directly across a joint was classified as direct invasion. Extension of tumors around a joint from its periphery to the opposing bone were considered as indirect invasion. RESULTS: All tumors were located near the sacroiliac joint, eight at the ilium and three at the sacrum. Six invasions were indirect and five were direct. Average tumor area was larger in indirect cases than in direct: 191.8 cm2 vs. 69.6 cm2. In all indirect invasions, a huge soft tissue mass abutted onto the peripheral portion of the sacroiliac joint. In five of six cases of indirect transarticular invasion, the upper part of the joint posteriorly located fibrous ligamentous cartilage. In the other, the lower part was invaded, and this involved a detour around the joint space, avoiding the invasion of intervening cartilage. Ankylosis occurred in one of the indirect cases. Among the five cases of direct invasion, there was invasion of the posteriorly located ligamentous fibrous cartilage in three without ankylosis. In the other two cases, involving ankylosis, the synovial hyaline cartilage was invaded directly at the lower part of the joint. CONCLUSION: Transarticular invasions of sacroiliac joint via fibrous cartilage are most common. Ankylosis of the sacroiliac joint facilitates hyaline cartilage invasion. We consider that in transarticular invasion of malignant pelvic bone tumors, indirect invasion is more common than direct.