Abstract Introduction Transition from paediatric to adult endocrinology is a challenge for adolescents, their families and their healthcare professionals. Previous studies show that up to 25% of young adults with endocrine disorders are lost to follow-up once they move out of paediatric care. This poses a health risk for young adults, as lack of medical treatment and surveillance can have both psycho-social and physical consequences. Apart from absenteeism from school or work, this can lead to serious and expensive medical complications like Addison crisis. Methods In 2019 we studied electronic medical records of 387 patients who were over 15 years old when they attended the paediatric endocrine outpatient clinic (OPC) of our medical centre in 2013-2014. We collected data from medical charts, the hospital digital agenda and medical correspondence. Results Of 387 adolescents, 161 (42%) did not need adult endocrine follow-up because paediatric endocrine care was only puberty- or growth-related. Forty-six patients did not enter regular transition because they 1) participated in a pilot to improve transition (N=10), 2) had intellectual disability (ID) and transferred to ID care (N=28), or 3) died (N=8, mostly cancer-related). Hundred-and-eighty patients entered regular transition: 49 (27%) to a regional hospital and 131 (73%) within our university hospital. Of these 131 patients, 33 (25%) were lost to follow up; in 24 of them (73%), the invitation for the adult OPC had never been sent. Loss to follow up occurred when three subsequent critical steps failed: 1) the adult endocrinologist had not received or read the paediatrician’s referral letter and/or had not invited the patient; 2) the paediatrician had not checked whether the appointment was really made and received by the patient and 3) the patients and/or caregivers had not alarmed the hospital when no invitation for an appointment was received. Conclusion We found a 25% dropout during transfer from paediatric to adult tertiary endocrine care. Starting the transition process early and in a structured manner, as well as assigning a transition coordinator, can prevent part of the dropouts. However, 73% of all dropouts appeared to be attributable to failure of practical, logistic steps. In order to prevent this part of the dropouts, we provide practical recommendations for all three parties involved: 1) the adult endocrinologist should carefully read paediatricians’ letters and check whether action is required (i.e. check whether an appointment is requested) 2) the paediatrician should ascertain whether the appointment is really made and received by the patient 3) the patients and/or caregivers should be instructed to alarm the hospital when they do not receive the appointment. These actions require relatively little effort and may prevent the part of drop-outs that is caused by logistic failures.