OBJECTIVE Nearly 50% of adults in Germany have no measurable antibody protection against diphtheria, even though most of them will have been vaccinated against it in their childhood. We investigated how vaccination should be performed in this situation to provide lasting protection. METHODS 100 adults (53 men, 47 women; mean age 27.7 [19-54] years) whose last diphtheria vaccination had been at least 10 years previously, were injected up to three times with customary commercial monovalent diphtheria vaccine for adults (5 IU toxoid/0.5 ml), at intervals of 4-8 weeks. Each time, before and 4-8 weeks after the latest injection, the antitoxin level was determined by cell-culture neutralization test. RESULTS Before the vaccination 30 of the 100 subjects had protection against diphtheria (antitoxin level > 0.1 IU/ml), 24 had "basic immunity" (0.01 to < 0.1 IU/ml), and 46 no measurable protection (< 0.01 IU/ml). After the first booster the protection threshold had not been reached in seven of the 100, all of them women. The main determinant for the pre- and post-vaccination antitoxin level was the interval since the last vaccination. In addition, women had significantly lower pre- and postvaccination levels than men. But after the second and third booster the antitoxin level of the seven women had risen to above the protection threshold, while the geometric mean of the whole collective had fallen, despite further boosters. It was 0.015 IU/ml before vaccination, 1.156 IU/ml after the first booster, 0.924 IU/ml after the second, and 0.952 after the third. CONCLUSIONS Those adults who were last vaccinated against diphtheria more than 20 years ago should have two booster shots, more than 8 weeks apart, so that the highest possible and most lasting antitoxin level can be achieved. Women in particular should have at least two booster shots, as their antitoxin response tends to be less than that of men.