We studied the levels of free radicals, lipid peroxides and antioxidants, as well as superoxide dismutase (SOD) activity in the blood of six patients with myotonic dystrophy (MyD) (mean age 52.8, SD 5.0 years) and seven controls (mean age 48.8, SD 6.3 years). Electron spin resonance was used to assess the free radicals by the spin-trapping method using 5,5-dimethyl-l-pyrroline-l-oxide. The levels of C centre radical (P < 0.05)="" and="" h="" radical="">P < 0.05)="" in="" blood="" from="" the="" six="" myd="" patients="" were="" significantly="" higher="" than="" those="" in="" the="" seven="" controls.="" the="" sod="" activities="" in="" red="" blood="" cells="" and="" serum="" from="" the="" six="" myd="" patients="" showed="" no="" significant="" difference="" from="" those="" in="" the="" seven="" controls.="" the="" serum="" lipid="" peroxide="" concentration="" was="" increased="" in="" five="" of="" the="" myd="" patients="" and="" tended="" to="" increase="" further="" as="" the="" disease="" progressed.="" the="" serum="" vitamin="" e="" level="" was="" low="" in="" two="" patients="" and="" in="" the="" low="" normal="" range="" in="" three.="" serum="" coenzyme="">₁₀ was decreased in four patients. The serum selenium level was decreased in two patients and that of serum albumin was decreased in three. Therefore we conclude that increased levels of free radicals and lipid peroxides and decreased antioxidant levels play an important role in the pathogenesis of MyD.