The aim of the study was to compare the validity and reliability of 2 sampling methods for measuring immunization rates to a reference standard in a national sample of pediatric office practices. The consecutive method involved patients seen consecutively in the office for any reason; the random record was a random selection of medical records; and the reference standard active method, data of a randomly selected subgroup of children in the random record survey were supplemented with information from a telephone interview. The consecutive method of assessing immunization rates results in rates that are, on average, higher and closer to the reference standard, but also more variable. The random record method rates are lower and further from the study reference standard compared with the consecutive method, but more precise. The consecutive method for measuring practice immunization rates could be a useful quality improvement tool as practices seek to improve immunization delivery and quality of care. It is inexpensive, simple, and easy to implement.