BACKGROUND: Immunization rates for children and adults are rising, but coverage levels have not reached national goals. As a result of low immunization rates, vaccine-preventable diseases still occur. In an era of increasing complexity of immunization schedules, rising expectations about the performance of primary care, and large demands on primary care physicians, it is important to understand and promote interventions that work in primary care settings to increase immunization coverage. A common theme across immunization programs in all nations involves the challenge of determining the denominator of eligible recipients (e.g., all children who should receive the measles vaccine), and identifying the best strategy to ensure high vaccination rates. Strategies have focused on patient-oriented interventions (e.g., patient reminders), provider interventions, and system interventions. One intervention strategy involves patient reminder/recall systems. OBJECTIVES: Assess the effectiveness of patient reminder/recall systems in improving immunization rates, and compare the effects of various types of reminders in different settings or patient populations. SEARCH STRATEGY: A systematic search was performed using MEDLINE (1966-1998) and 4 other bibliographic databases: EMBASE, PsychINFO, Sociological Abstracts, and CAB Abstracts. Authors also performed a search of EPOC in April 2001 to update the review. Two authors reviewed the lists of titles and abstracts, and used the inclusion criteria to mark potentially relevant articles for full review. The reference lists of all relevant articles and reviews were back searched for additional studies. Publications of abstracts, proceedings from scientific meetings, and files of study collaborators were also searched for references. SELECTION CRITERIA: Study Design: Randomized controlled trials (RCT), controlled before and after studies (CBA), and interrupted time series (ITS) studies written in English. Types of participants: Health care personnel who deliver immunizations and children (birth to 18 years) or adults (18 years and up) who receive immunizations in any setting. Types of interventions: Any intervention that falls within the Effective Practice and Organization of Care Group (EPOC) scope and that includes patient reminder and/or recall in at least one arm of the study. Types of outcome measures: Immunization rates, or the proportion of the target population up-to-date on recommended immunizations. Outcomes were acceptable for either individual vaccinations (e.g., influenza vaccination) or standard combinations of recommended vaccinations (e.g., all recommended vaccinations by a specific date or age). DATA COLLECTION AND ANALYSIS: Data Collection: Each study was read independently by two reviewers. Disagreements between reviewers were resolved by a formal reconciliation process to achieve consensus. Analysis: Results are presented for individual studies as relative rates for randomized controlled trials, and as absolute changes in percentage points for controlled before and after studies. Pooled results were presented using the random effects model. MAIN RESULTS: Patient reminder/recall systems were effective in improving immunization rates in 33 of 41 included studies, irrespective of baseline immunization rates, patient ages, type of setting, or type of vaccination. Increases in immunization rates due to reminders were in the range of 5 to 20 percentage points. Reminders were effective for childhood vaccinations (OR=2.02, 95% CI =1.49,2.72), childhood influenza vaccinations (OR=4.19, 95% CI =2.07,8.49), adult pneumococcus or tetanus (OR=5.14, 95%CI = 1.21, 21.8), and adult influenza vaccinations (OR=2.29, 95%CI = 1.69, 3.10). While reminders were most effective in academic settings (OR = 3.33, 95% CI = 1.98, 5.58), they were also highly effective in private practice settings (OR=1.79, 95% CI = 1.45, 2.22) and public health clinics (OR = 2.09, 95% CI = 1.42, 3.07). All types of reminders were effective (postcards, letters, telephone or autodialer calls), with telephone being the most effective but most costly. REVIEWER'S CONCLUSIONS: Patient reminder/recall systems in primary care settings are effective in improving immunization rates.