Children with monostotic and polyostotic bone dysplasias often exhibit localized bone overgrowth. We investigated the presence of nuclear estrogen and nuclear progesterone receptors by solid-phase radioimmunoassay, immunocytochemistry, and radioligand binding in osteoblast cell cultures derived from the areas of overgrowth of membranous bone, noninvolved membranous bone, and normal membranous bone from children undergoing elective craniotomy. Membranous bone of normal children had demonstrable levels of nuclear estrogen and progesterone receptors identified by radioimmunoassay and immunocytochemical assay. Two- to threefold increased levels of these receptors (p less than 0.001 versus normals) were found in cultures derived from the involved bone of two children with monostotic fibrous dysplasia and in one patient with polyostotic dysplasia (McCune-Albright syndrome). The noninvolved bone in our patients with fibrous dysplasia exhibited nuclear sex steroid hormone receptor levels similar to those in the normal children. Radioligand binding studies demonstrated increased sex steroid hormone receptors in cultures derived from involved osteoblasts. The presence of an increased level of sex steroid hormone receptor was accompanied by increased alkaline phosphatase activity and increased production of osteocalcin in vitro compared to normal or noninvolved bone. The mechanisms by which sex steroid hormone receptor levels are increased in the ostotic dysplasias remain to be established.