The initial development of the Toddler Temperament Scale (TTS) for determining temperamental characteristics in 1-to 3-year-old children is reported. The 97-item questionnaire was based on the conceptualization of Thomas, Chess, and associates in the New York Longitudinal Study and standardized on 309 children in two pediatric practices. Test-retest reliability (1 month) and alpha reliability are both satisfactory, with median correlations of .81 and .70, respectively, for the nine categories. Preliminary evidence of external validity is presented. The instrument should provide more objective and organized temperament data for research and clinical purposes.