The objective of this multinational open-label, prospective study was to collect, under naturalistic conditions, data on the effectiveness and tolerability of first-line monotherapy with valproate in subjects newly or recently diagnosed with focal onset epilepsy. Patients were treated with sustained release sodium valproate. Seizure control and occurrence of adverse events were assessed after 6 months. Around 1192 adults and 792 children were included. The mean daily valproate dose was 683 mg in children and 987 mg in adults. The retention rate at 6 months was 90.0%. At this time, 77% of subjects were seizure free (83.7% of children and 72.7% of adults). Adverse events possibly related to treatment were observed in 10.2% of subjects, leading to treatment modification for 1.7%. The most common adverse events were weight gain, gastro-intestinal, neurological and skin disorders. Sustained release sodium valproate is effective and shows acceptable tolerability as first-line monotherapy in focal onset epilepsy.