The objective of the study was to assess the efficacy and tolerability of sodium valproate (VPA) on chronic daily headache (CDH) in a prospective, double-blind, randomized, placebo-controlled trial. Seventy patients were included in the study. Twenty-nine had chronic migraine (CM) and 41 had chronic tension-type headache (CTTH). VPA and placebo were applied for 3 months to 40 and 30 patients, respectively. Visual analog scale (VAS) and pain frequency (PF) were used for evaluation. VPA decreased the maximum pain VAS levels (MaxVAS) and PF at the end of the study (P = 0.028 and P = 0.000, respectively), but did not change general pain VAS (GnVAS) levels (P = 0.198). In CM patients, the decreases in MaxVAS, GnVAS and PF parameters were more in VPA treated patients (P = 0.006, P = 0.03, and P = 0.000, respectively). VPA treatment caused more reduction in PF than placebo in the CTTH subgroup (P = 0.000). VPA is effective in the prophylactic treatment of CDH by reducing MaxVAS levels and PF. It was more effective in CM than in CTTH. © Springer-Verlag 2008.