Normal repair of skeletal muscle requires local expansion of a special population of Foxp3(+)CD4(+) regulatory T (Treg) cells. Such cells failed to accumulate in acutely injured muscle of old mice, known to undergo ineffectual repair. This defect reflected reduced recruitment of Treg cells to injured muscle, as well as less proliferation and retention therein. Interleukin-33 (IL-33) regulated muscle Treg cell homeostasis in young mice, and its administration to old mice ameliorated their deficits in Treg cell accumulation and muscle regeneration. The major IL-33-expressing cells in skeletal muscle displayed a constellation of markers diagnostic of fibro/adipogenic progenitor cells and were often associated with neural structures, including nerve fibers, nerve bundles, and muscle spindles, which are stretch-sensitive mechanoreceptors important for proprioception. IL-33(+) cells were more frequent after muscle injury and were reduced in old mice. IL-33 is well situated to relay signals between the nervous and immune systems within the muscle context. Copyright © 2016 Elsevier Inc. All rights reserved.