Introduction Inactivity is a major risk factor for premature mortality[1], as is reduced cardiorespiratory fitness (CRF) [2]and low levels of lean muscle mass (LMM) [3]. Despite this, less than one third of adults in Europe achieve the World Health Organisation (WHO) minimum weekly exercise targets [4], with “lack of time” a commonly cited barrier. High-intensity interval training (HIIT) is a proven time-efficient, feasible method to improve the CRF of older adults [5]. However, the majority of studies have employed HIIT in a supervised laboratory setting using specialised equipment. In order to assess the efficacy of HIIT for improving the CRF and LMM of older adults, with two known barriers to exercise (equipment and supervision) removed, we investigated the efficacy of 4-weeks supervised and unsupervised, equipment-free HIIT. Methods Twenty-one healthy older adults, median age 70 (range 61-81) years were randomised to complete 4-weeks laboratory HIIT (L-HIIT; supervised), home-based HIIT (H-HIIT; unsupervised) or a non-exercise control period. HIIT sessions lasted 15-minutes and comprised a brief warm-up, five 1-minute bouts of equipment-free high intensity efforts (star-jumps (2), static sprints (1) and squats (2)) interspersed with 90-seconds recovery and a final recovery period. The HIIT groups completed 12 sessions over 4-weeks. CRF, determined by VO2 peak and anaerobic threshold (AT), was assessed before and after the intervention period by a ramped cardiopulmonary exercise test (CPET), with LMM assessed by dual-energy X-ray absorptiometry (DXA). Results Both the L-HIIT (26.5±7.8 vs 28.3±8.6 ml/kg/min; p=0.04) and H-HIIT (21.2±3.3 vs 23.5±4.2 ml/kg/min; p=0.008) groups significantly improved their VO2 peak after HIIT, with no significant change in the control group (27.0±4.4 vs 26.8±3.8 ml/kg/min; p=0.99). Despite numerical increases in AT in both HIIT groups these increases failed to reach statistical significance (L-HIIT: 12.5±3.0 vs 13.7±3.0 ml/kg/min; p=0.13, H-HIIT: 11.8±1.1 vs 13.2±2.3 ml/kg/min; p=0.07), with no significant change in the control group (12.7±2.1 vs 12.2±1.4 ml/kg/min; p=0.77). Whole-body LMM significantly increased in both of the HIIT groups (L-HIIT: 48.9±9 vs. 49.6±10 kg; p=0.05, H-HIIT: 39.2±7 vs. 40±6 kg; p=0.03), with no significant change in the control group (46.2±7.7 vs 46.5±7.9 kg; p=0.69). Conclusion A 4-week, time-efficient, equipment-free HIIT programme is able to elicit improvements in the CRF and LMM of older adults. Furthermore, this programme is equally successful when performed unsupervised at home and as such may be useful in combating the rise of inactivity and associated poor health outcomes in adults. Larger studies are required to confirm these findings.