We aimed to examine the association of young and middle-aged sibling bereavement with use of health services and risk of developing mental health disorders over 24 months before and after sibling's death. All persons aged 18-56 years who died of illness/natural causes between 2009 and 2016 were identified in the Danish Register of Causes of Death. The study sample included 31,842 bereaved siblings and 31,842 age- and sex-matched controls. Overall, the largest effect was observed for increased use of psychological services: for women, the peak was observed four-six months after death (odds ratio [OR]=3.31, 95% confidence interval [CI] 2.85-3.85) and for men in the first three months after death (OR=2.63, 95%CI 2.06-3.36). The peak of being diagnosed in a hospitals setting with any mental disorder for women was observed in the period 13-15 months after sibling death (OR=1.52, 95%CI 1.11-2.07) and for men in the first three months after death (OR=1.75, 95%CI 1.32-2.32). Young and mid-aged adults who experienced the death of a sibling are more likely to use health services and are at risk of poorer mental health outcomes. During the bereavement process, young and mid-aged siblings are especially in need of mental health support, such as professional psychological counseling.