Objective: To assess total serum levels of coenzyme Q10 (Co-Q10), an important antioxidant, in children with cystic fibrosis (CF) and to investigate an association between Co-Q10 level and clinical outcome. Study design: Co-Q10 levels were measured annually in a prospective cohort study of 381 children with CF. A total of 1092 serum levels of total Co-Q10 were obtained by high-performance liquid chromatography and ultraviolet light detection. Associations of Co-Q10 with demographic variables and clinical outcomes were investigated. Results: Of the 381 initial total serum Co-Q10 measurements, 188 were in the deficient range. Low Co-Q10 was significantly more prevalent in patients with pancreatic insufficiency (PI) (55%) compared with patients with pancreatic sufficiency (PS) (3%); 22% of the patients with PI exhibited persistently low Co-Q10 levels. Low Co-Q10 levels were significantly associated with Pseudomonas aeruginosa colonization in patients with PI and CF under age 24 months, but not with subsequent lung function or hospitalization rates. Low Co-Q10 levels were related to other markers of nutritional status, including total lipids, beta-carotene, and alpha-tocopherol. Conclusions: Persistently low total serum Co-Q10 levels are common in children with CF and PI. A prospective study is indicated to determine whether Co-Q10 supplementation in CF is beneficial.