New Findings What is the central question of this study? We examined whether the macrophage‐synthesized antioxidant 7,8‐dihydroneopterin was elevated in Duchenne muscular dystrophy (DMD) patients. We then examined whether 7,8‐dihydroneopterin could protect dystrophic skeletal mouse muscle from eccentric contraction‐induced force loss and improve recovery. What is the main finding and its importance? Urinary neopterin/creatinine and 7,8‐dihydroneopterin/creatinine were elevated in DMD patients. 7,8‐Dihydroneopterin attenuated eccentric contraction‐induced force loss of dystrophic skeletal mouse muscle and accelerated recovery of force. These results suggest that eccentric contraction‐induced force loss is mediated, in part, by an oxidative component and provides a potential protective role for 7,8‐dihydroneopterin in DMD. AbstractMacrophage infiltration is a hallmark of dystrophin‐deficient muscle. We tested the hypothesis that Duchenne muscular dystrophy (DMD) patients would have elevated levels of the macrophage‐synthesized pterins, neopterin and 7,8‐dihydroneopterin, compared with unaffected age‐matched control subjects. Urinary neopterin/creatinine and 7,8‐dihydroneopterin/creatinine were elevated in DMD patients, and 7,8‐dihydroneopterin/creatinine was associated with patient age and ambulation. Urinary 7,8‐dihydroneopterin corrected for specific gravity was also elevated in DMD patients. Given that 7,8‐dihydroneopterin is an antioxidant, we then identified a potential role for 7,8‐dihydroneopterin in disease pathology. We assessed whether 7,8‐dihydroneopterin could: (i) protect against isometric force loss in wild‐type skeletal muscle exposed to various pro‐oxidants; and (ii) protect wild‐type and mdx muscle from eccentric contraction‐induced force loss, which has an oxidative component. Force loss was elicited in isolated extensor digitorum longus (EDL) muscles by 10 eccentric contractions, and recovery of force after the contractions was measured in the presence of exogenous 7,8‐dihydroneopterin. 7,8‐Dihydroneopterin attenuated isometric force loss by wild‐type EDL muscles when challenged by H2O2 and HOCl, but exacerbated force loss when challenged by SIN‐1 (NO•, O2•, ONOO−). 7,8‐Dihydroneopterin attenuated eccentric contraction‐induced force loss in mdx muscle. Isometric force production by EDL muscles of mdx mice also recovered to a greater degree after eccentric contractions in the presence of 7,8‐dihydroneopterin. The results corroborate macrophage activation in DMD patients, provide a potential protective role for 7,8‐dihydroneopterin in the susceptibility of dystrophic muscle to eccentric contractions and indicate that oxidative stress contributes to eccentric contraction‐induced force loss in mdx skeletal muscle.