Advance care planning (ACP) has been suggested to improve the quality of life (QoL) and mental wellbeing in severely ill patients and their relatives.To investigate the effects of ACP among patients with lung, heart and cancer diseases with an estimated life-span of up to 12 months.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.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.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.