Background: Advance care planning (ACP) enables persons to identify preferences for future treatment and care, and to discuss, record and review these preferences. However, the uptake of ACP among patients with chronic diseases is relatively low. Web-based ACP programs can support patients and their relatives in ACP. However, information needs of patients and their relatives for ACP are unknown. The aim of this study is to explore information needs of patients with chronic disease and their relatives for web-based ACP. Methods: We conducted semi-structured interviews with patients with chronic diseases and relatives at their home or at the study center. In three cases, the patient and relative were paired since they preferred to be interviewed together. We asked about information they would search for when to start with ACP, where they would search for information, what search terms they would use on the Internet, and what content and information they would consider important on an ACP website. The interviewer asked participants to clarify their responses during the interview. We used thematic analysis to analyze the interviewees' responses. Results: We interviewed nine patients with different chronic diseases including amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), chronic obstructive pulmonary disease (COPD) and kidney diseases, and seven relatives, namely partners or (adult) children. The interviewees were aged 24 to 80 years, nine were female and seven were male. Both patients with a chronic disease and relatives mentioned comparable information needs. Many interviewees indicated they would use the Internet to search for information about ACP. Mentioned search terms were "advance care planning", "treatment plan", "disease trajectory" and names of patient associations. Information needs concerned their disease trajectory and quality of life, medical treatment decisions, practical support in arranging care, the concept of ACP and guidance in ACP, communication of treatment and care preferences, peer support of others with chronic diseases, and information for relatives. Many appreciated encouragement of their healthcare providers to take a pro-active role in ACP. Conclusions: We conclude that information needs for ACP included guidance in ACP, support in making decisions about medical treatment, and practical support in arranging care. We recommend adapting web-based ACP information to the information needs of patients and their relatives to increase its findability, uptake and usefulness.