Several studies have demonstrated the slowing effect of corticosteroids on the decline of muscle strength in Duchenne muscular dystrophy (DMD). Deflazacort (DFC) is supposed to have fewer side effects than prednisone (PRED). An ongoing double blind multicenter study is comparing the effects and side effects of deflazacort (0.9 mg/kg/day) and prednisone (0.75 mg/kg/day) in DMD. This interim report includes data for 67 boys between age 5 years and loss of ambulation. Besides the common clinical and laboratory data for chronic corticoid treatment, motor performance has been tested. Interim results, 3-15 months after starting the medication, show some scattering but no grouping of data for all the functions tested: timed motor functions, sum of the strength of 20 muscles according to a 10-point scale on manual testing, weight gain, osteocalcin and alkaline phosphatase. Only the changes in CK activity after 3 months medication might reflect two equal groups without any correlation with the initial activity or with other parameters. On average, there was no clear-cut loss of muscle strength or performance. Except for in 4 patients, who were excluded due to unacceptable weight gain and/or loss of ambulation, there were no side effects considered to be serious. The results suggest that (i) DFC and PRED in equal anti-inflammatory dosage are similarly or equally efficient in slowing down the decline of muscle strength in DMD; (ii) benefits outweigh the side effects. This allows the study to continue as designed.