Preceded by an introduction to fibroblastic neoplasms and fibroblast biology, this chapter discusses lesions that include the word fasciitis in their name and two putative reactive lesions, retroperitoneal fibrosis and keloids. All of these are composed of fibroblasts and myofibroblasts; the latter are fibroblasts that typically contain smooth muscle actin and have contractile properties, originally described in wound healing.

Fasciitis is a clinicopathologically heterogeneous group that contains several distinct entities. Eosinophilic and necrotizing fasciitis are inflammatory and infectious conditions, respectively. They are unrelated to other lesions termed fasciitis but are included here because of similar terminology.

Benign fibroblastic tumors and tumor-like lesions form a large and heterogeneous group of non-neoplastic and neoplastic tumor entities. Some of these have a predilection for children, and some occur only in adults. Benign fibroblastic proliferations in this book are divided arbitrarily into four groups: (1) fasciitis and related lesions, (2) benign fibrous neoplasms (fibromas and related tumors), (3) fibromatoses, and (4) myxomas and other myxoid lesions. (Fibrous proliferations of children are presented in two separate chapters encompassing benign tumors and those of variable biologic potential.)

Lack of hard evidence makes it difficult to separate reactive and neoplastic conditions in some instances. Likewise, some lesions thought to be neoplasms could actually be reactive. In some instances, the commonly used designations are actually misleading about the true nature of the lesion; for example, nuchal fibroma, penile fibromatosis, and fibromatosis colli may all be reactive processes.