Self-reports of grade at first menstrual period for 1,967 fourth-grade through ninth-grade girls were used to categorize girls as early maturers. The categories of early maturer and other (on-time or late maturers) were then examined for stability over a 3-year period using McNemar tests and [kappa] coefficients. Although the results showed significant stability, 15% through 20% of the early maturing girls were eventually reclassified into the other category. This was true for Hispanic, Black, and White girls as well as for 1-, 2-, and 3-year time spans. Implications for measuring menarcheal timing and its effects are discussed. (Contains 3 tables and 1 note.)