Few studies have explored adoption of patient portals for pediatric patients outside primary care or disease-specific applications. We examined use of patient-provider messaging in a patient portal across pediatric specialties during the three years after implementation of pediatric portal accounts at Vanderbilt University Medical Center. We determined the number of patient-initiated message threads and clinic visits for pediatric specialties and percentage of these outpatient interactions (i.e., message threads + clinic visits) done through messaging. Generalized estimating equations measured the likelihood of message-based interaction. During the study period, pediatric families initiated 33,503 messages and participated in 318,386 clinic visits. The number of messages sent (and messaging percentage of outpatient interaction) increased each year from 2,860 (2.7%) to 18,772 (17%). Primary care received 4,368 messages (3.4% of outpatient interactions); pediatric subspecialties, 29,135 (13.0%). Rapid growth in messaging volume over time was seen in primary care and most pediatric specialties (OR>1.0; p<0.05).