Dysregulated wound healing after burn injury frequently results in debilitating hypertrophic scarring and contractures. Myofibroblasts, the main effector cells for dermal fibrosis, develop from normal fibroblasts via transforming growth factor beta 1 (TGF-beta 1). During wound healing, myofibroblasts produce extracellular matrix (ECM) proteins, modulate ECM stability, and contract the ECM using alpha smooth muscle actin (alpha-SMA) in contractile stress fibers. The antifibrotic pirfenidone has previously been shown to inhibit the initial differentiation of fibroblasts into myofibroblasts in vitro and act as a prophylactic measure against hypertrophic scar development in a mouse burn model. To test whether pirfenidone affects differentiated myofibroblasts, we investigated the in vitro effects of pirfenidone treatment after three to five days of stimulation with TGF-beta 1. In assays for morphology, protein and gene expression, and contractility, pirfenidone treatment produced significant effects. Profibrotic gene expression returned to near-normal levels, further alpha-SMA protein expression was prevented, and cell contraction within a stressed collagen matrix was reduced. These in vitro results promote pirfenidone as a promising antifibrotic agent to treat existing scars and healing wounds by mitigating the effects of differentiated myofibroblasts. (C) 2019 The Authors. Published by Elsevier Inc.