Objective: To evaluate the effectiveness of single-dose intramuscular methotrexate in the treatment of ectopic pregnancy at Maharaj Nakorn Chiang Mai Hospital Material and Method: A retrospective review of the patient records was performed on the patients with diagnosis of ectopic pregnancy and treatment with single-dose methotrexate according to the protocol of Stovall et al between 1996 and 2005. The successful treatment was defined as no need for surgical intervention. Results: As many as 96 out of 106 (90.6%) were successfully treated with methotrexate, though four required a second dose. Pretreatment β-human chorionic gonadotropin (β-hCG) levels were significantly lower in women who responded to single-dose therapy than either in those who required two doses or who had a failure of medical management (p < 0.01). The mean pretreatment level of β-hCG was 873 mlU/ml. The median time to resolution of β-hCG was 21 days. In addition, all cases with failed medical treatment had adnexal masses larger than 3.5 centimeters in diameter. Conclusion: In the present series, treatment of ectopic pregnancy with single-dose methotrexate was as high as 90% successful. Women with a high pretreatment β-hCG level and large sonographic adnexal masses had a greater probability of requiring either surgical intervention or multiple doses of methotrexate.