This study investigated whether emotional expression of traumatic experiences influenced the immune response to a hepatitis B vaccination program. Forty medical students who tested negative for hepatitis B antibodies were randomly assigned to write about personal traumatic events or control topics during 4 consecutive daily sessions. The day after completion of the writing, participants were given their first hepatitis B vaccination, with booster injections at 1 and 4 months after the writing. Blood was collected before each vaccination and at a 6-month follow-up. Compared with the control group, participants in the emotional expression group showed significantly higher antibody levels against hepatitis B at the 4 and 6-month follow-up periods. Other immune changes evident immediately after writing were significantly lower numbers of circulating T helper lymphocytes and basophils in the treatment group. The finding that a writing intervention influences immune response provides further support for a link between emotional disclosure and health.