Background : Diarrhea disease is still one important public health problem and a major contributor to morbidity third. Diarrhea diseases in the city of Yogyakarta is still a major health problem. Diarrhea become the number two cuse of death in children under five years. Dehydration caused by diarrhea is the leading cause of death in infants and children under five years. Cases of dehydration in children under five years is higher than infants. Dehydration will lead to health problem, starting from mild annoyances.The purpose of this study was to knowing the incidence diarrheal dehydration in children under five years at the RS PKU Muhammadiyah I Yogyakarta. Research Method :This study was an observational descriptive quantitative. The sampling technique used was consecutive sampling. Total population in this study were 460 population, a population of 460 were taken 10% or 46 respondents as sample. Result : Characteristics of the respondent’s age at most between the ages of 1 – 3 years as many 32 respondents (69,5%), female gender as much 25 respondents (54,3%), and male as much 21 respondents (45,7%), temperature respondents were normal (36,5 – 37,50C) of 23 respondents (50%) and hyperthermia (>37,50C) of 23 respondents (50%). Total incidence of dehydration in children under five years with diarrhea in the RS PKU Muhammadiyah I Yogyakarta most mild/moderate dehydration as much 31 respondents (67,4%), severe dehydration as much 5 respondents (10,9%), and without dehydration as much 10 respondents (21,7%). Conclusion : Total incidence of dehydration in children under five years with diarrhea in the RS PKU Muhammadiyah I Yogyakarta most are mild/moderate dehydration as much 31 respondents (67,4%), the majority aged 1 – 3 years as many 22 respondents (47,7%), female gender as much 16 respondents (34,8%), and hyperthermia temperature of 18 respondents (39,1%). Suggestion : There should be more research regarding the relationship between weight gain in children under five with the degree of dehydration