The influence of daily circadian rhythm on hepatitis B vaccination was investigated in medical student volunteers [Turkey]. First, 2nd and 3rd year medical students with hepatitis test results were randomized into morning and evening groups. 30 students (13 men and 17 women) were vaccinated in the morning between 08.00 and 08.30 h, and 33 students (14 men and 19 women) were vaccinated in the evening between 17.30 and 18.00 h. Hepavax-Gene vaccine was administered intramuscularly at the beginning, then 1 and 6 months later. Anti-HBs titres were measured quantitatively one month after the final vaccination. The results showed that after 3 doses of vaccination, the mean anti-HBs levels of the morning and evening groups were 923.3 and 921.2, respectively (P>0.05). The antibody responses in both groups were similar after application of 3 vaccine doses. It is concluded that circadian rhythm does not influence immune response against hepatitis B vaccination. Hepatitis B vaccine shots can be administered at any time of the day.