Archival records of US Masters Swimming performances were examined for changes between 1981 and 2021. Both national records and top 10 swimmers were used. Substantial secular changes were found averaging 0.52%/yr, with women improving more than men and with improvements in national records greater than in the top 10. Performances by women in 2021 were at parity (national records) or near parity (top 10) with men in 1981. The results indicate that secular effects must be considered along with longitudinal age-related changes and cross-sectional cohort effects in interpreting age differences in physiological function.