The purpose of this study is to determine if long-term treatment, up to 1 year, with Depakote ER is safe in the reduction of occurrence of migraine headaches in adolescents.

Inclusion Criteria:

• Subject and subject's legal representative have voluntarily signed and dated an IRB-approved informed assent form and an IRB-approved informed consent form, respectively, before any study-specific procedures or tests are performed;
• The subject was randomized into Study M02-488 and either completed the study or prematurely discontinued due to ineffectiveness; and
• The subject is male or non-pregnant, non-lactating female

Exclusion Criteria

• Female, of childbearing potential, and not using an effective method of birth control (e.g., total sexual abstinence or contraceptives).
• Experienced a serious adverse event in Study M02-488 which the investigator considered "possibly" or "probably related" to study drug; or
• In the investigator's opinion, long-term treatment with Depakote ER for migraine prophylaxis is contraindicated.
• For any reason, subject is considered by the investigator to be an unsuitable candidate to receive divalproex sodium or to participate in this study.