Conjunctivitis is a disease occurred around the world and may infect people regardless the age. Conjunctivitis is included as one of 10 diseases mostly suffered by outpatiens in Indonesia hospital (135.749 pasients). Conjunctivitis must be quickly addressed since it may lead to other complication depending on the etiology. A common complication of conjunctivitis is the loss of vision that will influence the patients in doing their daily activities. Objective: This research aimed to find out the level of middle adolescent knowledge about conjunctivitis in MAN 1 Yogyakarta Methods: This research was a descriptive research using survey as its approach. The sample used was 251 respondents with relative accuracy (d): 5%. The sample was collected through stratified random sampling. The data was collected by using questionnaire. The data was analyzed by descriptive statistic method. Results: The result indicated that level of middle adolescent knowledge in MAN 1 Yogyakarta in medium category was 115 peoples (45.8%), in good category was 96 peoples (38.2%) and in poor category was 40 peoples(15.9%). Conclusions and Recommendations: The level of middle adolescent knowledge about conjunctivitis at MAN 1 Yogyakarta was enough. It was expected that schools can provide a better counseling about conjunctivitis through school health program or trough the assistance of the Health Department in Yogyakarta