peer reviewed ; Seven persistency measures and their relationships with total, partial and peak yields were studied. Heritabilities, genetic and phenotypic correlations were estimated using mixed model equations with multiple trait restricted maximum likelihood (REML). Data were from 31,482 first lactation Belgian Holstein-Friesian cows. Differences in direction and size of the correlations between measures of persistency and total yield were found. Two types of measures were used. Persistency measures based on ratio showed positive correlations to total yield. Measures based on standard deviations of test, days or partial yields were correlated negatively with total yield. Correlations with partial yields also showed a similar pattern. Genetic correlations between ratio methods and peak yield were positive. Ratio methods not only describe persistency, but are highly influenced by total yield. Methods using standard deviations of test day yields showed a similar problem, but the influence of the level of total production was negative. Heritabilities of several persistency measures were above 0.10. After considering those results, a newly defined method, based on the standard deviation of partial yields, emerged as the best choice to describe persistency and for use in genetic evaluation of persistency.