To improve the pneumococcal vaccination status of an elderly patients group, those older than 64 years of age were identified from a computer file of all continuing care patients in a general internal medicine clinic. In a randomly chosen study group (N = 163), 91 elderly patients (56 per cent) had received the pneumococcal vaccine. Factors associated with a higher rate of pneumococcal vaccination included receiving the previous year's influenza vaccine, a medical problem list attached to the patient's chart, active clinic status (i.e., seen in the year before the study began), and more than two problems listed in the computer record. Letters encouraging pneumococcal vaccination were then sent to patients who had not been vaccinated. Twenty of 72 patients (28 per cent) who received the letter were vaccinated during the next year; 8 per cent of control patients (three of 39) who did not receive the letter were vaccinated. The 95 per cent confidence limit for the relative difference between the study and control group is 6 to 53 per cent. The relative difference was also significant for influenza vaccination between the intervention group and the portion of the control group that had not been vaccinated at the first chart review. Factors associated with pneumococcal vaccination rate following the mailing of the reminder letter were active clinic status and being up to date for either influenza or tetanus vaccination.(ABSTRACT TRUNCATED AT 250 WORDS)