Internet-based online patient-nurse communication (OPNC) services can constitute an important opportunity to support patients to manage their illness between treatment and rehabilitation while being at home. We explored the content of messages sent by prostate and breast cancer patients to an OPNC service to identify symptoms, problems, concerns and information needs expressed in these messages. Using qualitative content analyses we examined 276 messages sent from 38 breast and 22 prostate cancer patients during 15 months. Two main themes emerged: Concerns about physical symptoms and treatment side effects; and worries and questions about treatment and follow up. Analyses showed that cancer patients have many serious unanswered questions and concerns that can create considerable uncertainty and anxiety. An OPNC service can to a great extent meet patients' need for advice and information and thus be an important health care supplement that can improve quality of care.