Resistance exercise training (RET) can be applied effectively to increase muscle mass and function in older adults (65-75 years). However, it has been speculated that older adults above 85 years are less responsive to the benefits of RET. This study compares the impact of RET on muscle mass and function in healthy older adults 65-75 years versus older adults above 85 years. We subjected 17 healthy older adults 65-75 years (OLDER 65-75, n = 13/4 [female/male]; 68 +/- 2 years; 26.9 +/- 2.3 kg/m(2)) and 12 healthy older adults above 85 years (OLDER 85+, n = 7/5 [female/male]; 87 +/- 3 years; 26.0 +/- 3.6 kg/m(2)) to 12 weeks of whole-body RET (three times per week). Prior to, and after 6 and 12 weeks of training, quadriceps and lumbar spine vertebra 3 muscle cross-sectional area (computed tomography scan), whole-body lean mass (dual-energy X-ray absorptiometry scan), strength (one-repetition maximum test), and physical performance (timed up and go and short physical performance battery) were assessed. Twelve weeks of RET resulted in a 10% +/- 4% and 11% +/- 5% increase in quadriceps cross-sectional area (from 46.5 +/- 10.7 to 51.1 +/- 12.1 cm(2), and from 38.9 +/- 6.1 to 43.1 +/- 8.0 cm(2), respectively; p < .001; eta(2) = .67); a 2% +/- 3% and 2% +/- 3% increase in whole-body lean mass (p = .001; eta(2) = .22); and a 38% +/- 20% and 46% +/- 14% increase in one-repetition maximum leg extension strength (p < .001; eta(2) = .77) in the OLDER 65-75 and OLDER 85+ groups, respectively. No differences in the responses to RET were observed between groups (Time x Group, all p > .60; all eta(2) <= .012). Physical performance on the short physical performance battery and timed up and go improved (both p < .01; eta(2) >= .22), with no differences between groups (Time x Group, p > .015; eta(2) <= .07). Prolonged RET increases muscle mass, strength, and physical performance in the aging population, with no differences between 65-75 years and 85+ years older adults.