We assessed the validity and determined cut-off scores for the Finnish Autism Spectrum Screening Questionnaire (ASSQ). A population sample of 8-year-old children (n = 4,408) was rated via the ASSQ by parents and/or teachers, and a subgroup of 104 children was examined via structured interview, semi-structured observation, IQ measurement, school observation, and medical records. Autism spectrum disorders (ASDs) were diagnosed following DSM-IV-TR criteria. A search for hospital-registered ASDs was performed. For Finnish higher-functioning primary school-aged, 7- to 12-year-olds, the optimal cut-off score was 30 in clinical settings and 28 in total population screening using summed ASSQ scores of parents' and teachers' ratings. Determining appropriate cut-off scores in ASD screening in different languages and in different cultures is of utmost importance.