Background : Physical fitness is very important for students in supporting and facilitating their lecture activities. Late-level medical students need good physical fitness because they will take professional education with very dense learning activities and the risk of being exposed to various types of diseases during the learning process. Physical fitness is shown one of by cardiorespiratory fitness which shows the ability to take oxygen when doing physical exercise and reflects the metabolic abilities a person has. The body mass index is a simple measurement to assess a person's nutritional status. Fat accumulation can reduce physical fitness, although fat tissue is not directly involved in the process of energy formation. This study is needed to assess whether there is a relationship between fitness index and body mass index. Methods : This study was a cross-sectional study with data collection conducted at the FKIK UMY physiology laboratory. Respondents in this study who met the inclusion and exclusion criteria were 34 respondents. Data retrieval was done by measuring VO2max indirectly using the Harvard step test and body mass index measurements were carried out on respondents. Data analysis is done using a computer. Result : From the results of this study it was found that out of 34 respondents, 82.36% of respondents had a very poor fitness index. Body mass index results obtained as many as 74.7% of respondents in the normal range. The results of statistical tests obtained p=0,163 (p>0,05) where there was a relationship that was statistically insignificant in the respondents. Conclusion : There is no statistically significant relationship between cardiorespiratory fitness and the body mass index of late-level medical students.