The purpose of this study was to examine the effects of nine weeks of unilateral high volume, heavy resistance strength training (HRST) and 31 weeks of detraining on the three regions (proximal, middle, distal) of the quadriceps in young (25 yrs) and older (69 yrs) men and women. Quadriceps CSA was assessed as a difference of the trained leg minus the untrained leg (T-UT). A multi-way ANOVA with repeated measures revealed that after HRST, young men had significantly higher CSA (T-UT) compared to young women in all three regions (6.7 ± 1.4, 9.3 ± 1.4, 7.8 ± 1.2 versus 2.3 ± 1.5, 3.4 ± 1.5, 1.7 ± 1.3 cm2). Older men displayed significantly higher CSA (T-UT) in the proximal region compared to older women after HRST (6.4 ± 1.3 versus 2.4 ± 1.3 cm2). Both age groups had similar CSA (T-UT) values after HRST and after 31 weeks of detraining. Thus, age did not influence the magnitude of the increase in regional CSA (T-UT) after HRST nor did it influence the degree of loss after 31 weeks of detraining.