In this dose-ranging study 220 seniors who had received the 23-valent pneumococcal polysaccharide (PnPS) vaccine at least 5 years prior to enrollment were assigned to receive one of four volumes (0.1, 0.5, 1 or 2 ml) of 7-valent pneumococcal conjugate (PnC) vaccine or a 0.5 ml dose of 23-valent PnPS vaccine. All participants received a reduced challenge dose of 0.1 ml of PnPS vaccine 1 year after enrollment. There was evidence of a dose response to PnC vaccine and antibody levels in the 1 ml PnC group tended to be significantly higher than in the PnPS group. A booster response to the challenge vaccination was not observed. Administration of a 1 ml dose of PnC vaccine is more immunogenic than 0.5 ml of PnPS vaccine in elderly adults previously vaccinated with PnPS vaccine. © 2007 Elsevier Ltd. All rights reserved.