Introduction. Excessive fatness becomes a social problem. Often and often we can observe it in children and youth. Excess of fatty tissue can cause a lot of diseases and dysfunctions in whole body. This work is aimed at attempting to determine the effect of fatness and overweight on the lumbar spine motion. Objective. The aim of the research was an attempt of evaluation of fatness and overweight on spine motion especially on lumbar spine and first of all to give an answer to following research question: 1. Is there any correlation between maximal forward flextion and fatness depending on sex? Material and method. The research was carried out from January to February 2009 among 394 schoolchildren from primary school near Cracow. The subjects were group of 195 girls and 199 boys aged 7 to 12. The examination included measurement of BMI and sum of three skinfolds (subscapula skinfold, abdominal skinfold, triceps skinfold) in each child. Whole spine motion was examinated by fingers – floor test. The lumbar spine motion was examinated by Schober test. Results. The statistically significant correlation (p?0,001) between maximal spine flexion and fatness in whole examination group was observed. Correlation was statistically significant at p≥0,01 between fingers – floor test and fatness characterized by BMI and sum of three skinfolds. There were no differences between influence of fatness on maximal spine flexion in Schober test in connection with children sex. There were observed some differences between groups of sex in fingers – floor test. The coefficient of correlation were not statistically significant in girls, the level of significant up to the mark of 10% for sum of three skinfolds and 10% for BMI. Conclusions. Body fatness affects spine motion in primary school children.