Steinert's myotonic dystrophy (DM) is a genetic autosomal dominant disease and the most frequent muscular dystrophy in adulthood. Although causative mutation is recognized as a CTG trinucleotide expansion on 19q13.3, pathogenic mechanisms of multisystem involvement of DM are still under debate. It has been suggested that mitochondrial abnormalities can occur in this disease and deficiency of coenzyme Q 10 (CoQ10) has been considered one possible cause for this. The aim of this investigation was to evaluate, in 35 DM patients, CoQ10 blood levels and relate them to the degree of CTG expansion as well as to the amount of lactate production in exercising muscle as indicator of mitochondrial dysfunction. CoQ10 concentrations appeared significantly reduced with respect to normal controls: 0.85 +/- 0.25 vs. 1.58 +/- 0.28 microg/ml (p < 0.05). Mean values of blood lactate were significantly higher in DM patients than controls (p < 0.05) both in resting conditions (2.9 +/- 0.55 vs. 1.44 +/- 1.11 mmol/L) and at the exercise peak (6.77 +/- 1.79 vs. 4.90 +/- 0.59 mmol/L), while exercise lactate threshold was anticipated (30-50% vs. 60-70% of the predicted normal maximal power output, p < 0.05). Statistical analysis showed that serum CoQ10 levels were significantly (p < 0.05) inversely correlated with both CTG expansion degree and lactate values at exercise lactate threshold level. Our data indicates the occurrence of reduced CoQ10 levels in DM, possibly related to disease pathogenic mechanisms associated with abnormal CTG trinucleotide amplification.