Background and aims: Limited data exist on the effects of resistance training and detraining on functional performance in very old adults aged >80 years. First, to determine the effects of an 8-week resistance exercise program on muscle strength and functional performance in very old men. Second, to examine the effect of a 6-week detraining period in muscle strength and functional performance. Methods: Twenty-two men, aged >80 years, were randomized to three groups: resistance exercise-detraining group (RDT; n=8), resistance exercise group (RT; n=7) and control group (CON; n=7). RT and RDT groups performed an 8 week resistance exercise program for lower and upper muscle groups, two times a week. Thereafter, RDT underwent a 6-week detraining period, whereas the RT group continued resistance exercise. 3-RM strength, 6-minute walk distance (6-MWD), chair rising time, and Timed-Up and Go (TUG) tests were assessed at baseline, and at week 8 and week 14. Results: Significant improvements were observed in 3-RM strength (25%, to 55%; p<0.001) and functional performance (15 to 25%; p<0.001 ) tests, in RDT and RT after 8 weeks of exercise. RT continued to improve muscle strength and 6-MWD significantly (p<0.05) until the end of the exercise period, whereas significant declines in muscle strength (60 to 87%; p<0.05) and functional performance (36 to 70%; p<0.05) gains occurred during the exercise period were observed in RDT during the &week detraining period. No significant differences were observed in the control group. Conclusions: Results indicate that a resistance exercise program induces favorable muscular and functional adaptations in very old adults. However, a significant part of the favorable adaptations obtained after resistance exercise may be lost within 6 weeks of detraining. Therefore, very old adults should follow a long-term and systematic routine of exercise throughout life, in order to improve and maintain their physical functions and to ameliorate their life status.