The time to the appearance of a peak serum antibody response to influenza virus vaccine is not clearly defined. We compared the most commonly used time intervals described in the literature--4 and 6 weeks after vaccination. We studied 118 elderly patients from three different geographic sites. The 1992 to 1993 trivalent inactivated influenza virus vaccine containing influenza virus A/Beijing/353/89 (H3N2), influenza virus A/Texas/36/91 (H1N1), and influenza virus B/Panama/45/90 was used. No statistically significant differences were found at the 4- and 6-week intervals after vaccination.