To determine if high-dose pulsed dexamethasone is more effective and safer than daily high-dose prednisolone in treatment-naive adult patients with inflammatory myopathies (sporadic inclusion body myositis excluded) we performed a multicenter, double-blind randomised controlled clinical trial with 18 months follow-up. Sixty-two patients were randomised into 28-day cycles of oral high-dose dexamethasone or daily high-dose prednisolone. Primary outcome measures included (1) seven point composite score of six clinically relevant outcomes and (2) (time-to) remission and (time-to) relapse. No difference between both treatment groups on the composite score was found. Side-effects occurred significantly less frequently in the dexamethasone group. Median time to relapse was 60 (2.9) weeks in the prednisolone and 44 (4.7) weeks in the dexamethasone group (log-rank test p=0.03). In conclusion, pulsed high-dose oral dexamethasone is not superior to daily prednisolone as first-line treatment of idiopathic inflammatory myopathies, but is a good alternative by causing substantially fewer side-effects.