Introduction: L-test is a modification of the timed up and go test. Standing up and sitting down are in addition to walking important components of mobility. These tasks are relatively demanding for patients after lower-limb amputation, due to their reduced muscular capacity and proprioceptive information. For the assessment of the ability to stand up, the five times sit-to-stand (5TSTS) and the 30-second sit-to-stand test (30SSTS) are most commonly used. Purpose: To assess reliability of the L-test and its concurrent and discriminative validity, minimal detectable change and effect size. The aim was also to assess reliability of the 5TSTS and 30SSTS and to consider their usability in patients after lower-limb amputation in prosthetic phase of rehabilitation. Methods: 36 subjects (30 male, 6 female), mean age 64.0 (SD: 12.8) years, who underwent an inpatient rehabilitation and were provided with a prosthesis for the first time, participated in the study. Twenty-eight had trans-tibial and 8 trans-femoral amputation. They were amputated due to vascular disease (86 %) or other medical conditions. After the L-test we conducted 6-minute walk test, 10-meter walk test, 5TSTS and 30SSTS in randomized order. We conducted the first assessment on the day when patients were able to walk independently with prosthesis and the day after. All tests were conducted on the first assessment day and again after two and three weeks. We calculated values of the intraclass (ICC), Pearson’s (r) and Spearman’s (ro) correlation coefficients, t-tests for dependant and independent samples, effect size, minimal detectable change and floor and ceiling effects. Results: We established excellent intra-rater (ICC = 0.94) and inter-rater (ICC = 0.96) reliability for the L-test. Concurrent validity was moderate to excellent (r = 0.60–0.86). There was a significant difference in mean L-test results (F (1, 31) = 5.858; p = 0.022) between the subjects following trans-tibial and trans-femoral amputation. Regression analysis of the results of the L-test with respect to the level of amputation revealed an important total linear correlation with other variables (R2 = 0.55; p < 0.001). In this regard, statistically significant influences of the age of the subject, the cause of amputation and the walking aid were confirmed. L-test detected large differences after two weeks (effect size = 1.21). Minimal detectable change was 20.3 seconds; we did not detect the floor or ceiling effects. For 30SSTS and 5TSTS we established excellent intra-rater reliability (ICC = 0.92 and ICC = 0.93, consecutive) and detected the floor effect. With the modified performance of both tests (using hands) there was no floor effect. Conclusion: The L-test is a reliable measuring tool in patients after lower-limb amputation during the prosthetic phase of rehabilitation. Due to its good measurement properties, it is relevant to introduce it into regular physiotherapy practice. Both sit-to-stand tests are reliable and useful measures of functional mobility.