The Subcommittee on Reference Intervals of the National Committee for Clinical Laboratory Standards (NCCLS) has recently completed a proposed guideline, NCCLS Document C28-P, entitled "How to Define, Determine, and Utilize Reference Intervals in the Clinical Laboratory." This guideline document is an attempt to combine a concise set of procedures and recommendations, largely taken from the original literature, to form a standard, uniform, and reasonable protocol for determining population-based reference intervals. The intent of the guideline is to set forth the minimum requirements for the determination of a reliable and clinically useful reference interval. The subcommittee hopes the document will set a standard that upgrades the quality of reference intervals to a level worthy of their use in clinical medicine.