The term "Munchausen's syndrome by proxy" (MSBP) was first used in the 1970s to describe a potentially lethal variant of abuse. In representative cases a mother deceives physicians into treating her child for illness that she has fabricated or induced, her motivation being to accrue the intangible benefits of the "sick role." Increased efforts to identify and protect victims have sometimes resulted in misdiagnosis of MSBP, leading authorities to remove children from the home and/or bring criminal charges against an innocent parent. The authors believe that Improved child protection can be achieved by examining "what went wrong" in cases of erroneously diagnosed MSBP. A review of more than 200 articles and books yielded seven reports of misdiagnosed MSBP in the medical/surgical, psychiatric, nursing, social work, legal, and lay literature. These isolated reports are discussed, and four new cases are presented. The paper concludes with a discussion of lessons to be learned and a table showing situations in which the risk of an erroneous diagnosis of MSBP is increased.