With the recent development of neonatal medicine, the number of children with medical complexities (CMCs) is increasing. Outdoor activities are important for their psychosocial development, and the principles of accessibility should be addressed. We report the experience of 2 CMCs' high-altitude mountaineering with the necessary support. The participants were a 3-y-old girl with cerebral palsy, symptomatic epilepsy, and a ventriculoperitoneal shunt (Child A) and a 6-y-old girl who underwent bilateral Glenn operations at 11 mo for hypoplastic left heart syndrome (Child B). The support staff consisted of 4 doctors, 1 nurse, 5 nonmedical staff , 3 members from a mountaineering association, and 2 people from an oxygen company. The climbing schedule was 2 days. On the first day, we took a bus to a hut at an altitude of 2450 m and stayed overnight to acclimatize to the altitude. On the second day, we took the beginner's route, which took 3 h to climb 500 m, and our team made an attempt on the summit. During the attempt, Child B panicked. Although her lung sounds did not raise suspicions of pulmonary edema, we decided to leave the mountain with her because her transcutaneous oxygen saturation decreased. Child A had no apparent health problems and made it to the summit. Although CMCs' alpine climbing requires careful planning and staffing considering the risk of high-altitude sickness, our case suggests the feasibility of such activities with CMCs as part of accessibility.