On the basis of 3-week studies, lithium and valproate are both recommended for first-line treatment of acute mania. It is, however, also important to demonstrate that antimanic efficacy can be maintained. This study has been designed to compare the efficacy and tolerability of valproate and lithium over 12 weeks in the treatment of acute mania in patients with type I bipolar disorder. Three hundred patients with bipolar I disorder presenting with acute mania were randomized to open treatment with lithium (starting dose: 400 mg/day) or valproate (starting dose: 20 mg/kg/day) for 12 weeks. The primary efficacy criterion was remission (YMRS score ≤12 at study end and a reduction of ≥2 on the CGI-BP severity scale). Remission rates were 65.5% (lithium group) and 72.3% (valproate group). Noninferiority of valproate with respect to lithium was demonstrated [between-group difference: 6.78% (95% confidence intervals: -3.80 to 17.36%)]. Remission rates assessed by the secondary mixed model repeated measures analysis were significantly greater with valproate than with lithium. Adverse events were reported in 44% of patients in both groups. Valproate and lithium showed comparable efficacy and tolerability in the treatment of acute mania over 12 weeks.