Purpose: To present the design of a cluster randomized controlled trial (cRCT) to evaluate the effectiveness of a web-based intervention for improving provider knowledge about strabismus and amblyopia (S/A) and preschool vision screening (PVS), increase PVS rates, and improve rates of S/A diagnoses made by eye specialists. This is the first cRCT targeting amblyopia prevention. Methods: Participants were Medicaid providers in AL, SC, or IL who had Internet access and had filed at least 8 claims for well child visits (WCV) for children ages 3 or 4 years old during a 12-month period before enrollment. Randomization to the Intervention (vision) or Control (blood pressure) arm occurred at the cluster level, defined as the provider (or group of providers) and his/her patients seen for WCVs. Results: 65 Intervention providers (IPs) with 3547 children aged 3 or 4 years, and 71 Control providers (CPs) with 5053 children enrolled. The study will report measures of knowledge and self-reported vision screening behaviors from web-based data. The primary outcomes will be rates of PVS among PCPs, and rates of diagnosis of S/A by eye specialists among the children belonging to Control and Intervention practices. Conclusions: We had the same difficulty recruiting PCPs as reported by others. Baseline rates of PVS were low (14.1%), as were rates that S/A were diagnosed by eye providers (1.4%). Our data show a need to improve both primary outcome measures. © 2010 Elsevier Inc.