The immunogenicity of an acellular pertussis vaccine containing genetically detoxified pertussis toxin, filamentous haemoagglutinin and pertactin was studied in 12 children [median age: 45 (6-107) months] with perinatal human immunodeficiency virus-type 1 (HIV-1) infection. Antibody response to all antigens was observed in six cases and another children 3 reacted to two or one antigen(s), but titres were lower than those from healthy controls. Antibody titre fold-rise correlated with preimmunization CD4-positive cell counts. Significant titres were still detectable 4 months after the third dose. The acellular vaccine is immunogenic in a portion of children with perinatal HIV-1 infection but early vaccination might be more effective, taking advantage of still adequate CD4-positive cell numbers.