In a longitudinal, prospective study of patient's decisions about influenza vaccination, the stability of attitudes about the flu and flu shots, the stability of flu shot decisions, and the relationship of attitude shifts to compliance were studied. In both 1981 and 1982 for 216 patients at high risk for complications of influenza, attitudes about 15 issues in the decision to obtain a flu shot were measured and each patient's behavioral intention and flu shot behavior ascertained. From one year to the other, 53% of patients had at least two substantial attitude shifts, yet 91% of patients expressed the same behavioral intention, and 85% of patients had the same flu shot behavior. Reversals in flu shot decisions were closely related to shifts in attitudes concerning side effects of the flu shot, an association that was supported by other findings, including a marked difference in prevalence of previous side effects in shot takers (11%) versus nontakers (60%). The results suggest: intention reversals were less frequent than attitude shifts because only specific attitude changes about flu shots were associated with reversals, and interventions that induce positive attitude changes, especially about the side effects of flu shots, should be effective in improving flu shot compliance.