Pre- vs. post-vaccination changes in correlations between IgG concentrations (ELISA titres) and opsonophagocytic activity (OPA) against Streptococcus pneumoniae serotypes 6B, 14 and 23F induced by the 23-valent polysaccharide vaccine were studied in paired serum samples received from elderly individuals, haemodialysed patients and kidney transplant recipients by the Spanish Pneumococcal Reference Laboratory. The pre- and post-vaccination parameters considered were: ELISA and OPA titres and the percentage of subjects with post-vaccination OPA values above the cut-off levels; the correlations between OPA and ELISA (Spearman correlation coefficient, r); and the amount of IgG needed to obtain OPA (beta coefficient). Non-significant pre-vaccination correlations between OPA and ELISA were found. Vaccination increased the correlation coefficient between OPA and ELISA to a statistically significant level for serotypes 6B, 14 and 23F in samples from haemodialysed patients, for serotypes 14 and 23F in samples from elderly individuals, and for none of the serotypes in samples from transplant recipients. In all cases, except for serotype 23 in transplant recipients, vaccination increased the beta coefficient, indicating that lower amounts of IgG were needed to obtain high OPA titres. A globally lower response was obtained for serotype 23 and/or transplant recipients.