Objective: To evaluate the effects of pentoxifylline administration on the patients with different stages of endometriosis, for whom laparascopy was performed. Design: prospective, double-blind, randomized, placebo-controlled clinical. Design Classification: Canadian Task Force classification І. Setting: University and private hospitals. Patients: Eighty eight women, all with infertility, some with dysmenorrhea, dyspareunia, or pelvic pain, for whom laparascopic diagnosis of endometriosis was done and as the principal part of the treatment, appropriate surgery was carried out. Interventions: The treatment group received 800 mg of pentoxifylline daily for six months immediately after surgery. The control group received placebo capsules. All of them were followed for one year thereafter. Measurements and Main Results: A comparison of pregnancy rate and recurrence of signs and symptoms in the two mentioned groups was done. Forty three patients were studied in the pentoxifylline group, and forty five in the placebo one. The cumulative pregnancy rate was 39.5% and 35.6% in the treatment and control groups, respectively. The overall recurrence of signs and symptoms was 14% in the former group and 15.6% in the latter one. So there were no statistically significant differences between the two groups in the rates of pregnancy and recurrence (P values: .7 and .832, respectively). Neither there was any significant statistical difference between the same stages in the two groups regarding immunomodulation. Conclusions: According to the results of this study, and while keeping in mind that appropriate and perfect operation is the main aspect of the endometriosis treatment, there is no evidence that immunomodulation with pentoxifylline aids fertility, or recurrence of signs and symptoms in women with different stages of endometriosis (i.e., minimal, mild, moderate, or severe).