Reports of the phase III clinical trials on four combined progestogen-estrogen once-a-month injectable contraceptives, Deladroxate, Cyclofem, Mesigyna and Chinese Injectable No. 1, are reviewed focussing on efficacy and reasons for discontinuation. Deladroxate, currently used in many Latin American countries has proved to be highly effective and well accepted. However, this combination was withdrawn by the original manufacturer because the progestogen component of this drug induced a high number of breast cancers in dogs and very curious pituitary hyperplasia in rats. Cyclofem and Mesigyna were found to be highly effective and highly acceptable drugs. Side-effects were minimal and were of minor importance. The Chinese Injectable No. 1 had unacceptably high failure rates with a monthly injection schedule. After doubling the dose in the first month of use, the efficacy was satisfactory. It was found that all monthly injectable contraceptives provided better cycle control than the every 3 months depot-medroxyprogesterone acetate, although abnormal bleeding was still the main drug-related complaint and reason for discontinuation. Missed appointment is another reason for discontinuation which might reflect the problem of frequent injection schedule, thus indicating the need for proper selection of the users and good counselling.