BACKGROUND Immunization recall for specific vaccines may be necessary to "catch up" children with newly available vaccines or recall children after vaccine shortages. The extent to which immunization registry-based recall can increase immunization rates for a new vaccine has not been prospectively studied. OBJECTIVE To assess the efficacy of letter/telephone recall for immunization with pneumococcal conjugate vaccine (PCV7) in an economically disadvantaged urban population. METHODS Randomized controlled trial at an inner-city teaching hospital. Using an immunization registry, we randomly assigned children aged 6 weeks to 22 months unimmunized for PCV7 to an intervention group (n = 610) or a control group (n = 624). The intervention consisted of letter and telephone recall for PCV7 vaccination. Two months after intervention, receipt of 1 or more doses of PCV7 was assessed. Intention-to-treat analysis was used. RESULTS We were unable to successfully contact 42.3% of the intervention subjects by mail and telephone. In the intervention group, 23.0% (140 children) received PCV7 within 2 months compared with 20.2% (126 children) in the control group (P = NS). No intervention effect was evident when children were stratified by age. A large proportion of the study population had Medicaid insurance (51.2%) or were uninsured (28.5%), but response to PCV7 recall did not vary by insurance status. CONCLUSIONS Letter and telephone recall for PCV7 vaccine did not significantly increase the rate of PCV7 immunization in an inner-city teaching hospital serving a disadvantaged population. The effectiveness of recall appears to have been limited by the inability to reach many subjects by mail and telephone.