Investigators examined billing records from 4 university-affiliated primary care clinics for encounters with females aged 8-26 years from October 2007 to May 2008 to determine clinical service characteristics of initiating the human papillomavirus (HPV) vaccination series. A total of 5538 services were rendered to 1365 patients within 2550 visits. Sixteen percent of those who received HPV vaccination were adults. Statistically significant predictors of initiating the vaccination series included type of visit (ie, nurse visit and preventive visit) and the patient being a minor.