Few registries are available for evaluating population differences for rare, newly, or ill-defined pediatric neurologic disorders. The purpose of this article is to present standard methodologies for establishing a population-based registry and evaluating the completeness of a registry's case ascertainment. The Texas Rett Syndrome Registry (TRSR) is used as a model. The combination of health care and education resources has identified approx. 89-100% of the Rett syndrome cases in Texas. Cases reported by non-physician sources, although older on average (10.7 vs 7.7 years of age), did not differ by other demographic characteristics from those reported by physicians. Non-physician health and education professionals participated with the TRSR at a significantly higher rate than physicians, 89 and 37% (p < 0.05), respectively. Capture-recapture techniques, both two-sample and log-linear modeling, were used to quantitatively evaluate case ascertainment. Standardized national and international population-based registries could be the basis of an initiative to identify the etiology and perhaps preventive measures for pediatric neurologic disorders. © 1995.