Background: There has been a focus recently on the use of the Internet and email to deliver education interventions to general practitioners (GPs). The treatment of breast cancer may include surgery, radiotherapy, chemotherapy, and/or hormone treatment. These treatments may have acute adverse effects. GPs need more information on the diagnosis and management of specific adverse effects encountered immediately after cancer treatment. Objective: The goal was to evaluate an Internet-based educational program developed for GPs to advise patients with acute adverse effects following breast cancer treatment. Methods: During phase 1, participants viewed 6 video vignettes of actor-patients reporting 1 of 6 acute symptoms following surgery and chemotherapy and/or radiotherapy treatment. GPs indicated their diagnosis and proposed management through an online survey program. They received feedback about each scenario in the form of a specialist clinic letter, as if the patient had been seen at a specialist clinic after they had attended the GP. This letter incorporated extracts from local guidelines on the management of the symptoms presented. This feedback was sent to the GPs electronically on the same survey platform. In phase 2, all GPs were invited to manage similar cases as phase 1. Their proposed management was compared to the guidelines. McNemar test was used to compare data from phases 1 and 2, and logistic regression was used to explore the GP characteristics that were associated with inappropriate case management. Results: A total of 50 GPs participated. Participants were younger and more likely to be female than other GPs in Australia. For 5 of 6 vignettes in phase 1, management was consistent with expert opinion in the minority of cases (6%-46%). Participant demographic characteristics had a variable effect on different management decisions in phase 1. The variables modeled explained 15%-28% of the differences observed. Diagnosis and management improved significantly in phase 2, especially for diarrhea, neutropenia, and seroma sample cases. The proportion of incorrect management responses was reduced to a minimum (25.3%-49.3%) in phase 2. Conclusions: There was evidence that providing feedback by experts on specific cases had an impact on GPs' knowledge about how to appropriately manage acute treatment adverse effects. This educational intervention could be targeted to support the implementation of shared care during cancer treatment.