: The etiology of craniosynostosis remains unknown. The beta group of transforming growth factors (TGF-[beta]) and insulin-like growth factors (IGF-I and IGF-II) are known to induce new bone formation and, when added exogenously, cause accelerated closure of calvarial defects. The possible roles of these bone growth factors in premature cranial suture fusion in humans have not been explored. We analyzed a total of 20 cranial suture biopsy samples (10 synostotic and 10 normal) from 10 infants with single-suture craniosynostosis undergoing cranial vault remodeling. Using isoform-specific antibodies for TGF-[beta] 1,-[beta] 2, and-[beta] 3 and IGF-I, we demonstrated immunoreactivity of these growth factors in both the craniosynostotic and control suture samples. The results show that these bone growth factors were present in human cranial sutures; the TGF-[beta] 2 isoform was the most intensely immunoreactive. Most importantly, the TGF-[beta] isoforms and IGF-I showed more intense immunoreactivity in the actively fusing craniosynostotic sutures compared with the control patent sutures. Specifically, the TGF-[beta] isoforms and IGF-I were intensely localized in the osteoblasts synthesizing new bone at the suture margin. It is noteworthy that although the patent sutures were less immunoreactive for TGF-[beta] isoforms than fused sutures, there was a distinct pattern of the TGF-[beta] 3 isoform that was immunolocalized to the margin of the normal patent sutures. This suggests a possible role for TGF-[beta] 3 in maintaining cranial suture patency. The increased immunoreactivity of both TGF-[beta] 2 and IGF-I in the actively fusing sutures compared with the patent control sutures indicates that these growth factors may play a role in the biology underlying premature suture closure. To our knowledge, this is the first study showing the presence of TGF-[beta] 1,-[beta] 2, and-[beta] 3 and IGF-I in prematurely fusing human cranial sutures. In the future, manipulating the local expression of these growth factors at the suture site may enable plastic surgeons to modulate premature suture fusion.