Purpose <p>This study explored preferences for electronic health (eHealth) information support regarding chemotherapy adverse reactions among young and middle-aged breast cancer patients based on the Kano model.</p> Methods <p>This cross-sectional study recruited eligible breast cancer patients from two tertiary hospitals in China. The participants completed a general information questionnaire, the Patient-Reported Outcomes version of the Common Terminology Criteria for Adverse Events subset, the electronic Health Literacy (eHEALS), and the eHealth information support demand attribute questionnaire based on the Kano model. Data were analyzed using latent profile analysis and binary logistic regression.</p> Results <p>A total of 388 patients were enrolled. Latent profile analysis classified young and middle-aged breast cancer patients undergoing chemotherapy into two classes: “a low adverse reaction–symptom adaptation group” (C1, 76.3%) and “a high adverse reaction–symptom distress group” (C2, 23.7%). In C1, eHealth demands comprised two one-dimensional attributes, five attractive attributes, and three indifferent attributes. In contrast, all ten identified eHealth demands in C2 were categorized as attractive attributes. Five attractive attributes were common to both groups.</p> Conclusions <p>Young and middle-aged breast cancer patients exhibited distinct preferences for eHealth information to support concerns regarding chemotherapy adverse reactions. Tailoring eHealth information recommendations to these preference profiles may better address their diverse informational needs, help mitigate adverse reactions, and ultimately improve their quality of life.</p>