Objective: To determine how cardiac patients use the Activeheart.dk digital toolbox Methods: Mixed-methods study. A total of n = 33 cardiac surgical patients were enrolled in a study, which encouraged use of the Activeheart portal for 4 weeks after discharge. Patients were surveyed with respect to their demographic characteristics, e-health literacy skills, use of the Internet and use of the portal. In addition to the questionnaire survey, qualitative interviews were conducted with six randomly selected patients in order to study users’ experiences with and use of the portal. Results: Quantitative results showed no relation between cardiac patients’ prior e-health literacy skills and their use of the portal. Approximately 25 out of 31 patients stated that they had the skills to seek, locate and use health information on the Internet. Among the 17 patients who completed the first and second questionnaires, nine rarely used Activeheart.dk, four used it several times per month, and four several times per week. In 10 out of 16 replies, Activeheart.dk was rated as relevant or very relevant. Themes from the qualitative interviews revealed that the effects of medication and emotional, physical and cognitive changes had a higher impact on the use of the portal than prior e-health literacy skills. Conclusion: While participants’ e-health literacy skills ratings were high, and while participants stated that the content of the portal was relevant, their actual use of the portal remained limited. No relationship was found between participants’ e-health literacy skills and their use of the portal. Factors such as patient motivation, resources and timing of the intervention’s introduction seemed to be of greater importance.