Background and Objectives: Failure to thrive in early years of life increases risk of physical growth retardation and probability of behavioral, emotional and cognitive problems in later period of life. Therefore it is considered to be one of the major concerns of child health. The objective of this cross-sectional study was to determine the prevalence of failure to thrive in 6-36 months old children at health and medical centers affiliated to Asadabadi region of Tabriz. Materials and Methods: In this cross-sectional study, implemented in February and March, 2000, a total of 1559 children (781 males, 778 females) were randomly selected using family folder numbers and their weight and height were measured. Percent of weight and height for age was calculated in proportion to ideal body weight, height (50th percentile on WHO/NCHS-World Health organization / National Center for Health Statistics - growth chart) for their age and sex. More than 90% was considered as normal, between 90-75% mild, 75-60% moderate, less than 60% as severe growth failures. Results: On the basis of weight for age parameter and comparing with NCHS growth chart 1.5% of children (1.4% males, 1.6% females) had severe to moderate, and 16.4% (14.5% males, 18.5% females) showed mild failure to thrive. Comparing with common growth chart for girls and boys, severe and moderate failure to thrive were observed in 1.7% of children (1% males, 2.3% females), while mild in 17.7% children (10.2% males, 25.3% females). Percentage of girls and boys found above 50th percentile was 57% and 55% respectively.Conclusion: Considering the results of this study, usage of separate growth chart for girls and boys are recommended.