Ethoxyquin (EQ) is used as a synthetic antioxidant mainly in animal food, but it has also been reported to have neuroprotective properties by diminishing the peripheral neuropa-thy resulting from anti-cancer chemotherapy in animal models. However, EQ has not been thoroughly evaluated in humans and its use is under debate. The objective of this work was to examine the effect of EQ on primary fibroblasts derived from controls and patients with confirmed mitochondrial dysfunction. To this end, we incubated primary fibroblasts from patients with mitochondrial respiratory chain complex 1 (C1) deficiency, cyto-chrome-c-oxidase (COX) deficiency and genetic form of Parkinson’s disease (PD). Alt-hough EQ did not alter ATP production it significantly increased oxygen consumption and improved energy status in C1 and COX cells while decreasing ROS production in COX, PD and control cells and maintaining mitochondrial content. These effects could possibly be attributed to the antioxidant and chaperone-modulating properties of EQ and to induced SOD2 expression in some cells. Our results indicate that EQ has the potential to improve some mitochondrial functions. Still, caution should be exerted since overdose of this com-pound affected cell growth.