Proteus syndrome is a complex disorder comprising malformations and overgrowth of multiple tissues. The disorder is highly variable and appears to affect patients in a mosaic manner. This intrinsic variability has led to diagnostic confusion associated with a dearth of longitudinal data on the natural history of Proteus syndrome. To clarify some of these issues, a workshop on Proteus syndrome was held in March 1998 at the National Institutes of Health, and participants developed recommendations for diagnostic criteria, differential diagnosis, and guidelines for the evaluation of patients. This is a review of those recommendations.