Introduction and Aim: The main objective of the research is to determine the level of knowledge of fifth grade girls about menstruation and hygiene before the implementation of health education on the same subject. The specific objective of the research is to prove the justification of formal education on this subject by the health professionals. Material and methods: The research was conducted on pupils of fifth grade in doctor's office of the school of medicine in Kaštel Sućurac. The research included 36 respondents. The research was conducted in February 2016 in the doctor's office of the school of medicine in Kaštel Sućurac, in collaboration with a doctor and nurse School of Medicine, before the hour of health education on the topic of "Period and hygiene". The research was conducted through a questionnaire of twenty nine questions, which is made using manual "O tebi". Collected data from the questionnaire were entered into a Microsoft Excel table to the code previously prepared plan. For processing is used descriptive statistics. Results: The most represented age categories of respondents was 11 years. The respondents were aged 12 years (N = 3), 10 years (N = 8) and 11 years (N = 25). More than half of respondents correctly stated the first sign of puberty in girls. A total of 8% of the respondents of this research were given period. One participant got her period age of 10 years, and two participants have received period age of 11 years. More than half of the respondents answered that testes is not external part of the female genitals, while 39% gave an incorrect answer. 89% of respondents correctly identified the body parts that do not belong to the inner part of the female genitals. Ejecting the eggs from the ovary, 58% respondents considered that is the period, and 17% respondents considered that is the ejaculation. The menstrual cycle is the period between two periods is considered 83% respondents. Period lasts between 3 and 6 days, 78% is considered, while 11% is considered to be between 1-10 days. Total 86% respondents considered that the hair may wash during the period. Conclusion: Results of the research indicate medium high level of knowledge respondents of fifth grade about period and hygiene. A total of 75% respondents answered incorrectly the question of ovulation and a way of recognizing the correct answer, no matter what they are taught in biology class; 14% respondents said that must not be washed hair during period, 11% respondents said that must not take a shower during period. The research was conducted on a small number of respondents, so we can get a real picture of knowledge and education in girls fifth grade. Should be carried more detailed research on a larger number of respondents to be able to make a definitive conclusion about the knowledge of respondents about period and hygiene.