Background: Advance care planning (ACP) has been suggested to improve the quality of life (QoL) and mental wellbeing in severely ill patients and their relatives. Aim: To investigate the effects of ACP among patients with lung, heart and cancer diseases with an estimated life-span of up to 12 months. Methods: Patients and relatives were randomised into two groups: one receiving usual care and one receiving ACP and usual care. Themes from the ACP discussion were documented in patients' electronic medical file. Participants completed self-reported questionnaires four to five weeks after randomisation. Findings: In total, 141 patients and 127 relatives participated. No significant differences were found according to outcomes. However, patients with non-malignant diseases had the highest level of anxiety and depression; these patients seemed to benefit the most from ACP, though not showing statistically significant results. Conclusion: No significant effects of ACP among patients with lung, heart, and cancer diseases and their relatives regarding HRQoL, anxiety, depression, and satisfaction with healthcare were found.