An analysis of 80 thin-sections of the long bones of a relatively large and presumably adult specimen of Confuciusornis sanctus, a basal bird from the Early Cretaceous of China, reveals osteohistological patterns similar in some ways to those of extant birds of approximate size, but different in other ways. The fibrolamellar tissue typical of its cortex is commonly laid down by adult birds at standard estimates of 10 μm or more radial growth per day. If this rate were characteristic of its ontogeny, Confuciusornis could have reached maturity in 20 weeks or less. These tissue patterns, read straight, suggest lower growth rates than in the larger immediate relatives of Confuciusornis, the coelurosaurian dinosaurs, as well as in large extant birds, taxa that reached their large adult size relatively quickly. However, tissues of the inner cortex, as well as new analyses of rates in living variants of the fibro-lamellar tissue complex, suggest far faster growth and an estimated time to maturation as short as 8–13 weeks, longer than in most small living birds but commensurate with larger birds or relatively slowly growing birds such as tinami.