Patients with hematological diseases undergoing diagnostic or therapeutic splenectomy are at increased risk of pneumococcal infections. Vaccination is a straightforward option in preventing these infections. A well-defined cohort of splenectomized patients with hematological disorders was followed according to response to 23-valent pneumococcal capsular polysaccharide (Pneumovax N) vaccination. A total of 76 splenectomized patients (Hodgkin lymphoma, HL 26, non-Hodgkin lymphoma, NHL 19, immune-mediated cytopenias 28, and others 3) with a median age of 52 years (range 18-82 years) were included. Pneumococcal polysaccharide (PS) antibodies were determined using an enzyme-linked immunosorbent assay before vaccination, at peak, and follow-up. A poor response to vaccination was observed in 21 (28%) patients and a good response in 55 (72%), respectively. During the follow-up period of 7.5 years (range 3.5-10.5 years) after vaccination, and despite repeated revaccination in many cases, a total of five episodes (in three patients) of pneumococcal infections were reported, all confined to the poor responder group. Revaccination did not improve antibody levels in this group. The median age at vaccination was significantly higher in the group of poor responders (p=0.0006). None of the following factors could predict a poor antibody response: gender, disease activity or aggressiveness in hematological malignancies, previous radiotherapy and/or chemotherapy, time between splenectomy and pneumococcal vaccination, time between chemotherapy/radiotherapy and study pneumococcal vaccination (1 year), or the presence of hypogammaglobulinemia. In conclusion, a substantial proportion of splenectomized patients with hematological diseases mounted a poor PS antibody response and remained at risk for pneumococcal infections despite vaccination. In the absence of apt indirect clinical predictors of antibody response, with the exception of age, measurement of antibody levels seems to be a feasible method for early identification of this patient subgroup. Poor responders do not benefit from revaccination, and should be offered other prophylactic measures.