Six patients with interstitial pregnancies were treated with systemic or local injections of methotrexate, 15 mg i.m. daily for 5 days, or 1 mg/kg for 1 day. One dose of folinic acid rescue (50 mg) was administered on the first day of the treatment course. Diagnosis of interstitial pregnancy was established either by laparoscopy or transvaginal ultrasound. Out of six patients, five had serial measurements of serum human chorionic gonadotrophin (HCG), progesterone (P) and 17 beta-oestradiol (E2) until either the ectopic pregnancy resolved or surgery was performed. For one patient operated on day 1 after medical treatment, no serial serum measurements were performed. Serum HCG became undetectable under medical treatment in only four of the six patients. Out of these four patients, three had an initial level of HCG less than 1000 mIU/ml. Two patients underwent surgery (salpingectomy) because either the level of serum HCG did not decrease after the course of methotrexate therapy or it was required the next day to stop haemorrhage. In these patients, the initial level of HCG at the time of diagnosis, was 5300 and 43,000 mIU/ml, respectively. In the four patients who received conservative medical treatment only, the next menstrual period occurred 20-46 days after the onset of methotrexate and was preceded by luteal activity. A control hysterosalpingography performed 2 months later showed that in the four patients who received medical treatment only, the Fallopian tube was patent, and three became pregnant within 1 year of the methotrexate therapy. One of two patients who failed to respond to medical treatment and required surgical treatment, became pregnant 6 months later.(ABSTRACT TRUNCATED AT 250 WORDS)