The purpose of this study was to examine the effects of detraining on fitness performance in 7-year-old children after 8 weeks of muscular fitness training, which took place during the first 15 minutes of regularly scheduled physical education (PE) class. Children from 2 PE classes were cluster randomized into either an exercise group (n = 20) or a standard PE control group (n = 19). Performance on the long jump, single-leg hop, curl-up, and balance test was assessed at baseline, after training, and after an 8-week detraining period. A significant interaction of group by time after training was observed in the exercise group with improvements noted in abdominal curl-up and single-leg hop performance (p < 0.05). After detraining, the exercise group maintained training-induced gains on the curl-up (group mean [95% confidence interval] posttraining of 27.9 [21.2-34.5] to detraining 27.3 [21.1-33.6] repetitions; p < 0.05) and single-leg hop (posttraining 79.8 [73.2-86.4] to detraining 79.7 [73.0-86.5] cm; p < 0.05). Conversely, long jump (posttraining 113.8 [108.2-119.5] to detraining 110 [102.6-117.5] cm; p < 0.05) regressed toward baseline values in both groups. After detraining, balance performance (1.5 [1.3-1.7] seconds) regressed relative to baseline (2.0 [1.7-2.4] seconds) and posttraining (2.0 [1.8-2.4] seconds; p < 0.05). These findings indicate that the phenomenon of detraining in children is complex and characterized by different adaptations and regressions in strength, power, and balance. Regular participation in fitness activities during PE may be needed to enhance and maintain performance in all measures of muscular fitness in 7-year-old children.