We evaluated the immune status against diphtheria (D), tetanus (T) and polio viruses (PV) and the immune response to re-administration of the respective vaccines in a series of 23 transplanted homozygous beta-thalassemic patients, aged 5-17 years (mean age 12.1 +/- 3.1 years). They had been given compulsory DT toxoids and types 1, 2 and 3 PV vaccine in infancy and had been successfully submitted to allogeneic BMT 2-6 years previously. Prior to revaccination, a high percentage of subjects (from 48% for type 2 PV to 83% for D) had antibody levels below the protective levels and low geometric mean titers (GMTs). After revaccination (three doses of DT toxoids and of inactivated PV vaccine) the percentage of subjects with protective levels of antibodies rose to 86-100% and the GMTs increased markedly. We conclude that: (1) the protection afforded by compulsory DT and PV vaccines administered in infancy is almost entirely lost in beta-thalassemic patients for several years after BMT, (2) revaccination is necessary in these subjects, and (3) at least three doses of DT and PV vaccines must be administered to recover adequate protection.