BACKGROUND: There are few standardised assessments of unilateral muscular endurance and little data examining the validity and reproducibility of isokinetic unilateral muscular endurance performance measures. OBJECTIVE: To examine the validity and reproducibility of an isokinetic assessment of lower limb unilateral muscular endurance performance.METHODS: Thirty individuals (15 endurance trained and 15 non-endurance trained) repeated a time to exhaustion (TTE) based endurance protocol on separate days. Procedures were conducted on an isokinetic dynamometer and involved leg extension through 135° of the dominant leg. The TTE protocol required participants to perform unilateral isokinetic leg extensions at 60% of individual maximal voluntary contraction (MVC) until the specified intensity and frequency could not be maintained. RESULTS: The endurance trained individuals reached significantly greater TTE than their non-endurance trained counterparts (P<0.001). Systemic bias was small (<5%) and measures were similar between trials. Test-retest reproducibility statistics were as follows; CV: 6.9%; LOA −170.1 to 139.5; RLOA: −0.320 ×/÷ 0.257. Relative reproducibility statistics were tight between trials (significant correlation P<0.001).CONCLUSIONS: Data demonstrate the validity and tight test-retest reproducibility of the aforementioned assessment of unilateral muscular endurance performance.