Efforts to determine the prevalence of serious emotional disturbance in preschool-aged children have been hampered by the lack of a validated measure. The Preschool and Early Childhood Functional Assessment Scale (PECFAS) is a multi-dimensional measure that assesses the psychosocial functioning of children aged 3-7 years. The concurrent validity and reliability of the PECFAS were assessed in a sample of 30 preschool-aged children in a large Head Start program in Ventura, California. PECFAS ratings based on in-depth interviews were significantly related to parental ratings that the children had mental health problems, psychiatric diagnoses, teacher ratings of the child's need for mental health evaluations, teacher ratings of behavior problems on a standardized screening inventory (DIAL-R), and actual referrals for mental health evaluations. Interrater reliability for the total PECFAS score was high (r = .90) as was internal consistency of the five subscales (alpha = .86). Using the PECFAS scores as a standard, the weighted prevalence of serious emotional disturbance in this West Coast Head Start program was 17%, at the lower end of the current estimated rate of SED for older children in low income samples (18-26%).