Decentralizing health services, including those for HIV prevention and treatment, is one strategy for maximizing the use of limited resources and expanding treatment options; yet few methods exist for systematically identifying where investments for service expansion might be most effective, in terms of meeting needs and rapid availability of improved services. The Nigerian Government, the United States Government under the President's Emergency Plan for AIDS Relief (PEPFAR) program and other donors are expanding services for prevention of mother-to-child transmission (PMTCT) of HIV to primary health care facilities in Nigeria. Nigerian primary care facilities vary greatly in their readiness to deliver HIV/AIDS services. In 2012, MEASURE Evaluation assessed 268 PEPFAR-supported primary health care facilities in Nigeria and developed a systematic method for prioritizing these facilities for expansion of PMTCT services. Each assessed facility was scored based on two indices with multiple, weighted variables: one measured facility readiness to provide PMTCT services, the other measured local need for the services and feasibility of expansion. These two scores were compiled and the summary score used as the basis for prioritizing facilities for PMTCT service expansion. The rationale was that using need and readiness to identify where to expand PMTCT services would result in more efficient allocation of resources. A review of the results showed that the indices achieved the desired effect—that is prioritizing facilities with high need even when readiness was problematic and also prioritizing facilities where rapid scale-up was feasible. This article describes the development of the two-part index and discusses advantages of using this approach when planning service expansion. The authors' objective is to contribute to development of methodologies for prioritizing investments in HIV, as well as other public health arenas, that should improve cost-effectiveness and strengthen services and systems in resource-limited countries.