The diagnostic validity of the new research algorithms of the Autism Diagnostic Interview–Revised and the revised algorithms of the Autism Diagnostic Observation Schedule was examined in a clinical sample of children aged 18–47 months. Validity was determined for each instrument separately and their combination against a clinical consensus diagnosis. A total of N = 268 children (n = 171 with autism spectrum disorder) were assessed. The new Autism Diagnostic Interview–Revised algorithms (research cutoff) gave excellent specificities (91%–96%) but low sensitivities (44%–52%). Applying adjusted cutoffs (lower than recommended based on receiver operating characteristics) yielded a better balance between sensitivity (77%–82%) and specificity (60%–62%). Findings for the Autism Diagnostic Observation Schedule were consistent with previous studies showing high sensitivity (94%–100%) and alongside lower specificity (52%–76%) when using the autism spectrum cutoff, but better balanced sensitivity (81%–94%) and specificity (81%–83%) when using the autism cutoff. A combination of both the Autism Diagnostic Interview–Revised (with adjusted cutoff) and the Autism Diagnostic Observation Schedule (autism spectrum cutoff) yielded balanced sensitivity (77%–80%) and specificity (87%–90%). Results favor a combined usage of the Autism Diagnostic Interview–Revised and Autism Diagnostic Observation Schedule in young children with unclear developmental problems, including suspicion of autism spectrum disorder. Evaluated separately, the Autism Diagnostic Observation Schedule (cutoff for autism) provides a better diagnostic accuracy than the Autism Diagnostic Interview–Revised.