Chronic lymphocytic leukaemia (CLL) is a common adulthood mature B-cell neoplasm. Infections are the most important cause of mortality in this condition, and Streptococcus pneumoniae has been considered the most important single pathogen. We investigated the immunogenicity of 7-valent pneumococcal conjugate vaccine in patients with CLL. The study material comprised 52 patients with CLL and 25 age- and sex-matched controls. The subjects were vaccinated with Prevenar pneumococcal conjugate vaccine. Serum samples were taken for antibody determinations before and four weeks after vaccination. Antibody response rates to vaccine antigens were lower in patients with CLL compared to controls. However, if the vaccine had been administered at an early stage of the disease, i.e. before commencement of chemotherapy and the development of hypogammaglobulinaemia, a significant vaccination response to at least six antigens was obtained in almost 40% of the CLL patients. Our results indicate that early administration of conjugate vaccine may be beneficial in CLL.