OBJECTIVE To evaluate the safety, convenience and effectiveness of medical treatment of hemodynamically stable tubal pregnancy using intramuscular methotrexate even with adnexal masses up to 5 cm in diameter. STUDY DESIGN A prospective, observational study was performed on patients admitted with hemodynamically stable tubal pregnancy to the Fourth Unit, Department of Obstetrics and Gynecology, Ain Shams University Maternity Hospital, from September 1999 to August 2000, and fulfilling the inclusion and exclusion criteria. All were given intramuscular methotrexate, 50 mg/m2. RESULTS Thirty-five (66.04%) of 53 patients admitted with tubal pregnancy were eligible for the study. Seventeen patients (48.5%) had an adnexal mass 3.6-5.0 cm in diameter by transvaginal ultrasonography. Thirty-three patients (94.3%) were cured, with 25 (75.8%) requiring a single dose of methotrexate, 7 (21.2%) two doses and 1 (3%) three doses. Two treatment failures (5.7%) occurred, and both had an adnexal mass > 3.5 cm. The mean time for the hCG level to return to normal was 34.8 days (range, 15-70). Of the 33 patients cured, 20 (60.6%) were treated on an outpatient basis, 7 (21.2%) needed brief readmission due to severe separation pain, and 6 (18.2%) were hospitalized for logistic reasons. There was a large statistically significant difference in the serum hCG level when a cutoff level of 1,000 mIU/mL was used to compare those with an adnexal mass < or = 3.5 cm and those with a mass 3.6-5.0 cm as well as those needing more than one dose and those needing one (P < .001). Similarly, there was a statistically significant difference in the number of doses needed between those with an adnexal mass < or = 3.5 cm and those with a mass 3.6-5.0 cm (P < .05). However, multivariate analysis failed to show any statistically significant relation between treatment failure, hCG level, mass size, gestational age, or number of doses due to small sample size and limited number of events. CONCLUSION Intramuscular methotrexate for hemodynamically stable tubal pregnancy, even in cases with adnexal masses up to 5 cm in diameter, is safe and effective. Larger trials are needed to validate this approach.