OBJECTIVE: The goal of this study was to evaluate the effect of postoperative residual tumor size on the outcomes for patients with incompletely resected benign meningiomas who underwent radiotherapy (RT). METHODS: Fifty-four patients with incompletely resected benign meningiomas received postoperative RT between 1984 and 1995. Surgery consisted of partial resection for 43 patients (80%) and biopsy for 11 (20%). All patients underwent postoperative imaging (using computed tomography and/or magnetic resonance imaging), and the median residual tumor size was 3.5 cm (in greatest dimension). Thirty-eight tumors (69%) were <5 cm, and 17 (31%) were > or =5 cm. The median RT dose was 54 Gy (range, 45-60 Gy), delivered in daily fractions of 1.8 to 2 Gy. The median follow-up period was 55 months. RESULTS: The 5-year actuarial progression-free survival (PFS) rate for the entire group was 76%. The only significant predictor of PFS rates was residual tumor size. Large residual tumors (> or =5 cm) exhibited a worse 5-year PFS rate than did small tumors (<5 cm) (40 versus 93%, P < 0.0001). When analyzed as a continuous variable, residual tumor size remained a significant prognostic factor. Age, sex, tumor histological features, tumor location, timing of treatment (immediate versus delayed), extent of resection, and RT dose (<54 Gy versus > or =54 Gy) did not reach prognostic significance. The difference in PFS rates for small and large residual tumors translated into a significant difference in 5-year cause-specific survival rates (65 versus 97%, P = 0.01). CONCLUSION: For incompletely resected benign meningiomas treated with RT, residual tumor size is the most significant predictor of tumor control. Small residual tumors are well controlled with conventional RT doses and techniques. In contrast, more aggressive therapies should be considered for large tumors.