Background Low cardiorespiratory fitness (CRF) is associated with the development of cardiovascular diseases during childhood, adolescence and older ages. The purpose of the study was to investigate associations between fatness, hemodynamic characteristics and secondary time with CRF in primary school-aged children. Methods Height, weight, body mass index (BMI), blood pressure (BP), heart rate (HR), CRF (20 m shuttle-run) and sedentary time were measured for 105 children (categorized as normal, overweight, obese). The independent sample t-test checked for differences and one-way ANOVA-Post Hoc Test and stepwise linear regression analysis assessed the 20 m shuttle-run performance predictors. Results There was a statistically significant difference in CRF between boys and girls. There was a statistically significant difference between (p < 0.05) CRF for Normal weight (M = 47.58 +/- 3.26 kg m(-2)) and Obese (M = 44.78 +/- 3.23 kg m(-2)). CRF correlated with age, BMI and sedentary time (r > 0.3; p < 0.05). BMI is the best independent predictor of CRF. Conclusions Children with normal BMI tend to present better CRF performance than obese and overweight children. Sedentary behaviour is associated with lower CRF in primary school-aged children.