Background: Concern about side effects is a barrier to influenza vaccination. This randomized, double-blind, placebo-controlled trial assessed side effects following vaccination among healthy working adults. Methods: Healthy working adults were recruited during October and November 1994 and were randomized to receive influenza vaccine or placebo injections. Local and systemic symptoms during the week following the injection were evaluated through structured telephone interviews. Results: Of 849 subjects enrolled in the study, 425 received a placebo and 424 received influenza vaccine. Baseline characteristics were similar between the groups, and 99% of subjects completed interviews to assess side effects after the study injection. No differences were seen between the 2 groups for the systemic symptoms of fever, myalgias, fatigue, malaise, or headaches. Overall, 35.2% of placebo and 34.1% of vaccine recipients reported at least 1 of these systemic symptoms (P=.78, χ2). Vaccine recipients reported a higher rate of arm soreness at the injection site than did placebo recipients (63.8% vs 24.1%, P<.001). Local reactions were mild in both groups and infrequently resulted in decreased use of the arm. After logistic regression, female sex (odds ratio [OR], 1.5; 95% confidence interval [CI], 1.1-2.1), age younger than 40 years (OR, 1.6; 95% CI, 1.2-2.2), and coincidental upper respiratory tract illness (OR, 4.6; 95% CI, 3.26.6) were independently associated with higher rates of systemic symptoms. In the multivariate model, vaccine again was not associated with systemic symptoms (OR, 0.9; 95% CI, 0.7-1.2). Conclusions: Influenza vaccination of healthy working adults is not associated with higher rates of systemic symptoms when compared with placebo injection. These findings should be useful to physicians and other health care providers as they counsel patients to take advantage of an important opportunity for disease prevention and health protection.