One of the more controversial issues in terms of mental disorder diagnoses has been their differential sex prevalence. The conclusions provided in the 4th edition of the American Psychiatric Association's (1994) Diagnostic and Statistical Manual of Mental Disorders were derived from systematic reviews of the research literature. However, this research is perhaps complicated by common sources of error. Two sources, in particular, are emphasized here: biases in sampling and biases within the diagnostic criteria themselves. The potential for such biases is illustrated for a wide variety of mental disorder diagnoses, and suggestions for research to address them are provided.