Objective To evaluate effect of en bloc resection of malignant sacral tumor involving partial sacrum and ipsilateral sacroiliac joint with reservation of contralateral sacral foramina.Methods Between February 2005 and July 2010,16 patients with malignant sacral tumor involving partial sacrum and ipsilateral sacroiliac joint underwent hemi-sacrectomy and functional reconstruction.The operation achieved en bloc resection of tumor with reservation of contralateral sacral foramina.There were 10 males and 6 females,aged from 17 to 70 years (average,37.9 years).There were 5 cases of chondrosarcoma,3 cases of Ewing's sarcoma,2 cases of malignant schwannoma,2 cases of osteosarcoma,1 case of malignant giant cell tumor of bone,1 case of lymphoma and 2 cases of metastatic tumor.Results The surgical time ranged from 4 to 11.5 hours (average,6.5 hours).The average blood loss was 3600 ml (range,1500 to 5500 ml).Five patients underwent type Ⅰ resection,while 11 patients underwent type Ⅱ resection.All patients were followed up for 21 to 59 months (average,34.4 months).Three months after operation,13 patients had difficulty with ankle plantar flexion,while sphincter function was more or less reserved in 16 patients.Four patients (25%)had wound healing complications,which was cured after debridement and drainage.Local recurrence occurred in 7 patients.There were 5 cases of local recurrence among 6 patients who had intralesional margin;4 recurrence lesions located in the sacrum and 1 in soft tissue.Eight patients (50.0%) survived without tumor,2 patients (12.5%)survived with tumor,and 6 patients (37.5%) died of tumor.Conclusion The en bloc resection of tumor involving partial sacrum and ipsilateral sacroiliac joint with reservation of contralateral sacral foramina was feasible and had better outcome compared with total sacrectomy. Key words: Sacrum; Sacroiliac joint; Bone neoplasms; Treatment outcome