Investigated whether the association of small numbers with the left and of large numbers with the right space in Western cultures is already present in preschoolers and second-graders. 30 preschool children (mean age 6 years) and 29 children in second grade (mean age 8 years) estimated the position of numbers on a number line that was labeled either with a small number on the left side and a large number on the right side (i.e., left-to-right orientation) or vice versa (i.e., right-to-left orientation). Results show that estimations were less accurate if the number line was oriented from right to left and if the children started with this condition. The performance of second graders, too, was affected by the orientation of the number line, but to a smaller degree than that of preschoolers. It is concluded that preschooler already possess a culture-typical spatial-numerical association that develops before the acquisition of reading and writing skills and that significantly affects the accuracy of numerical estimations.