Features of Parkinson's disease (PD) include oxidative stress, nigral mitochondrial complex I deficiency and visual dysfunction, all of which are also associated with coenzyme Q(10) (CoQ(10)) deficiency. The objective of this monocenter, parallel group, placebo controlled, double-blind trial was to determine the symptomatic response of daily oral application of 360 mg CoQ(10) lasting 4 weeks on scored PD symptoms and visual function, measured with the Farnsworth-Munsell 100 Hue test (FMT), in 28 treated and stable PD patients. CoQ(10) supplementation provided a significant (P=0.01) mild symptomatic benefit on PD symptoms and a significantly (F((1,24))=8.48, P=0.008) better improvement of FMT performance compared with placebo. Our results indicate a moderate beneficial effect of oral CoQ(10) supplementation in PD patients.