Purpose: To evaluate the long-term outcome and prognostic factors in patients with skull base erosion from nasopharyngeal carcinoma after initial radiotherapy (RT). Methods and Materials: From January 1985 to December 1986, 100 patients (71 males, 29 females) with a diagnosis of nasopharyngeal carcinoma were found on computed tomography (CT) to have skull base erosion. The mean age was 41 years (range 16-66). Ninety-six patients had World Health Organization type III undifferentiated carcinoma, and 4 had type I. The metastatic workup, including chest radiography, liver ultrasound scanning, and liver function test was negative. All patients underwent external beam RT (EBRT) alone to 66-80 Gy during 6-8 weeks. A daily fraction size of 2 Gy was delivered using 60Co or a linear accelerator. No patient received chemotherapy. All patients were followed at regular intervals after irradiation. he median follow-up was 22.3 months (range 2-174). Survival of the cohort was computed by the Kaplan-Meier method. The potential prognostic factors of survival were examined. Multivariate analyses were performed using the Cox regression model. Results: The 1, 2, 5, and 10-year overall survival rate for the cohort was 79%, 41%, 27%, and 13%, respectively. However, the subgroup of patients with both anterior cranial nerve (I-VIII) and posterior cranial nerve (IX-XII) involvement had a 5-year survival of only 7.7%. A difference in the time course of local recurrence and distant metastasis was observed. Both local recurrence and distant metastasis often occurred within the first 2 years after RT. However, local relapse continued to occur after 5 years. In contrast, no additional distant metastases were found after 5 years. The causes of death included local recurrence (n=59), distant metastasis (n=21), both local recurrence and distant metastasis (n=1), and unrelated causes (n=5). After multivariate analysis, complete recovery of cranial nerve involvement, cranial nerve palsy, and headache after irradiation were found to be independent prognostic factors in this cohort. Conclusions: We present one of the longest follow-ups of patients with nasopharyngeal carcinoma invading the skull base. Our results demonstrate the importance of cranial nerve involvement, recovery of headache, and cranial nerve palsy. These factors should be carefully evaluated from the history, physical examination, and imaging studies. A subgroup of patients with skull base involvement had long-term survival after RT alone. The findings of this study are important as a yardstick against which more aggressive strategies, such as combined radiochemotherapy and altered fractionation RT can be compared.