OBJECTIVES: To scrutinise claims about the effectiveness of the Standard Days Method® (SDM). The SDM is a calendar method with similarities to the rhythm method that has now been reclassified and is marketed as a modern contraceptive method. As promoted, it requires users to avoid unprotected intercourse on days 8-19 of the menstrual cycle. It is used in at least 100 countries. SDM has been researched, developed, and is marketed by the Institute of Reproductive Health (IRH) at Georgetown University, USA, and a for-profit company Cycle Technologies. SDM proponents say it is a major advance on traditional periodic abstinence, claiming that it is 95% effective when used correctly - rivalling pills and condoms. The effectiveness claim is repeated in recent documents from the World Health Organization. STUDY DESIGN: Evaluation of evidence obtained via literature review of published and unpublished reports. RESULTS: Claims made for SDM effectiveness appear to rely on a single efficacy study where "correct use" of SDM was defined as total abstinence from intercourse in cycle days 8-19. It may therefore be misleading to apply a 95% effectiveness figure from the study to SDM as promoted, where abstinence is not required. Moreover, "typical use" effectiveness figures, cited as 88%, are based on an unrepresentative sample of women using SDM in ways likely to vary from how SDM is used in practice. CONCLUSION: Existing evidence does not support claims that the effectiveness of SDM as promoted is comparable to the best short-acting modern contraceptive methods. SDM is promoted in ways that may mislead users, by quoting overestimates of effectiveness and providing efficacy comparisons only with selected methods of contraception. Users should be provided with full and accurate information to make contraceptive choices. IMPLICATIONS: Use, delivery and promotion of SDM should be reevaluated. Meanwhile, SDM should only be offered to family planning clients as an adaptation of traditional periodic abstinence methods, requiring total abstinence in fertile days - reflecting "correct use" in the efficacy study - to achieve high effectiveness. Delivery of any form of SDM should include presentation of the full range of other contraceptive methods, including the most effective options.