Given a growing emphasis on early intervention for children with autism, valid quantitative tools for measuring treatment response are needed. The Social Responsiveness Scale (SRS) is a brief (15-20 minute) quantitative measure of autistic traits in 4-to 18-year-olds, for which a version for 3-year-olds was recently developed. We obtained serial SRS measurements on 73 preschool children with (n = 51) and without (n = 22) autism spectrum conditions. Inter-rater reliability (mothers and teachers) and test-retest reliability were of the order of 0.75 (Pearson's r). There was substantial agreement between SRS scores and (1) the Vineland Adaptive Behavior Composite (Pearson's r = -0.86) and (2) scores for social impairment on the Autism Diagnostic Interview-Revised (r = 0.63). Overall, quantitative autistic trait scores tended to improve over time in preschoolers, irrespective of treatment conditions. We conclude that it is possible to obtain reliable quantitative measurements of autistic social impairment in preschoolers, suitable for assessing treatment response.