OBJECTIVES/HYPOTHESIS The primary objectives of this study were to determine the intrasubject reliability of repeated measures of maximum phonation time (MPT) duration during a single session and to examine the effects of age, sex, and total phonatory airflow on this reliability. STUDY DESIGN This study used repeated measures. METHODS Duration and total phonatory airflow during three consecutive MPT trials were collected from 20 participants evenly distributed between age (young/old) and sex (male/female) groups. Intraclass correlation coefficient and repeated-measures analysis of variance were used to examine the reliability of MPT across trials and to test for possible effects of age and sex. RESULTS Intraclass correlation for MPT duration was strong across all participants (0.86), but MPT duration was not stable across trials (F2, 32 = 3.58, P = 0.04), with the second trial having the longest duration on average (P = 0.03). There was no effect of trial on total phonatory airflow (F2, 32 = 1.08, P = 0.35). The relationship between MPT duration and total phonatory airflow, however, did not remain consistent across trials (F2, 31 = 3.58, P = 0.04). There were no effects of age or sex on any variables. CONCLUSIONS The variability in MPT duration across trials and the inconsistent relationship between MPT duration and total phonatory airflow indicate that there is variability in laryngeal efficiency across repeated MPT trials. Therefore, the results of this study corroborate previous research supporting the assertion that MPT is not a reliable measure of laryngeal aerodynamics.