Proteus syndrome is a rare and highly variable hamartomatous syndrome that can affect multiple organ systems. It is characterized by hyperplastic lesions of connective tissue, vascular malformations, linear verrucous epidermal nevi, and hyperostoses. The cause of the disorder is unknown, but the current working hypothesis is that it is caused by a mosaic alteration that leads to a highly variable phenotype, equal sex ratio, sporadic occurrence, and discordant monozygotic twins. Herein we describe our experience with 18 patients with a referring diagnosis of Proteus syndrome. It was found that imaging studies are very useful for the characterization of the syndrome. One finding was that splenic hyperplasia can be a manifestation of Proteus syndrome. Analysis of the clinical data shows that Proteus syndrome is frequently confused with "hemihyperplasia." A distinct subtype of hemihyperplasia is defined that includes static or mildly progressive hemihyperplasia and multiple lipomata.