INTRODUCTION: We report the reliability of a new measure, the triple-timed up-and-go (3TUG) test, for assessing clinical function in patients with Lambert-Eaton myasthenia (LEM). METHODS: Intrarater reproducibility and interrater agreement of the 3TUG test were assessed in 25 control participants, 24 patients with non-LEM neuromuscular disease, and 12 patients with LEM. The coverage probability (CP) method was the primary measure of reproducibility and agreement. The a priori acceptable range was < 20% difference in 3TUG test times and a CP ≥0.90 confirmed agreement. RESULTS: CP values > 0.90 for intrarater and interrater tests confirmed acceptable reproducibility and agreement for all groups. DISCUSSION: The 3TUG test is a quick, noninvasive, and reproducible measure that is easy to perform, measures clinically important weakness in LEM patients, and requires little training. Additional evaluation in a larger number of LEM patients is in progress to validate the 3TUG test as a clinical measure in LEM. Muscle Nerve 57: 136-139, 2017.