To provide standards for the diagnosis and treatment of patients with hydatidiform mole and gestational trophoblastic tumours (GTT).Prognostic factors useful for treatment decisions in GTT are defined with patients classified as low-, medium-, and high-risk groups.Improved mortality and morbidity.Evidence was gathered using Medline for relevant studies and articles from 1980 to 2001 with specific reference to diagnosis, treatment options, and outcomes. The quality of evidence of Recommendations has been described using the Evaluation of Evidence criteria outlined in the Report of the Canadian Task Force on the Periodic Health Exam.1. Suction curettage is the preferred method of evacuation of the hydatidiform mole (III-C). Post-operative surveillance with hCG assays is essential (II-3B). 2. Low-risk patients with both non-metastatic and metastatic disease should be treated with single-agent chemotherapy, either methotrexate or dactinomycin (II-3B). 3. Medium-risk patients should usually be treated with multi-agent chemotherapy, either MAC or EMA (III-C); single-agent chemotherapy may also be used (III-C). 4. High-risk patients should be treated with multi-agent chemotherapy EMA/CO, with selective use of surgery and radiotherapy (II-3B). Salvage chemotherapy with EP/EMA and surgery should be employed in resistant disease (III-C). 5. Placental site trophoblastic tumour that is non-metastatic should be treated with hysterectomy (III-C). Metastatic disease should be treated with chemotherapy, most commonly EMA/CO (III-C).6. Women should be advised to avoid pregnancy until hCG levels have been normal for six months following evacuation of a molar pregnancy and for one year following chemotherapy for gestational trophoblastic tumour. The combined oral contraceptive pill is safe for use by women with GTT (III-C).These guidelines have been reviewed and approved by the Policy and Practice Guidelines Committee of the Society of Obstetricians and Gynaecologists of Canada (SOGC), the Gynaecologic Oncologists of Canada (GOC), the Society of Canadian Colposcopists (SCC), and by Executive and Council of the SOGC.The Society of Obstetricians and Gynaecologists of Canada.