Meningococcal conjugate vaccines are today successfully deployed in universal programs for children and adolescents in different geographic regions to control meningitis and septicemia. However, in adults, the advantages of these conjugates over the older polysaccharide vaccines are less clear. In this randomized clinical trial, we demonstrated that both conjugate and polysaccharide quadrivalent meningococcal vaccines elicit protective antibody responses in adults aged 18 to 70. (This study has been registered at www.clinicaltrials.gov under registration no. NCT00901940.).