This study was designed as a longitudinal study with the purpose of investigating the effects of providing mouth care education to pediatric oncology patients on the degree of oral mucositis. The study sample included 16 children aged 8 to 18 years who were hospitalized in the pediatric oncology and hematology clinics at a university hospital. The results revealed a statistically significant difference between the degree of mucositis before and after the education given to children undergoing chemotherapy (P < .05). The median pain values were significantly different before and after the education (P < .05) as well. It was also found that there was a strong positive statistically significant correlation between the degree of mucositis and mean pain score both before and after the education (P < .001). Consequently, it is reported that both the degree of mucositis and pain levels decreased when children were given planned mouth care education before chemotherapy and when they regularly performed mouth care.