Objective: To determine the effect of pre-school experience on social maturity of first grade elementary children in Iran. Methods: The descriptive cross-sectional study was conducted in Kermanshah, Iran, during 2006-7. Using random sampling, 62 elementary school children were selected for the study. The students were assessed on the Social Maturity Scale comprising 117 questions that measured social maturity using 8 sub-scales. The face validity of the questionnaire was tested using a panel of experts from the Department of Psychology, Kermanshah Medical University. After reviewing the instrument, appropriate adjustments were made to the questionnaire. Reliability test involved pilot study among 20 children outside the study population. Cronbach's alpha coefficient was used to test for the 8 sub-scales. SPSS 14 was used for statistical analysis. Results: Of the 62 children, 31 (50%) had been to pre-school. Compared to the other set of 31 (50%), those with preschooling developed more social maturity, scoring 70-72 on the social maturity scale while the others scored 69-71 (p0.042). Result of this study has implications for families and elementary school officials. Conclusion: The difference between those with pre-schooling and those without it was significant, but not so highly significant as to recommend making it mandatory. (author abstract)