Prior studies suggest autism-specific and general developmental screens are complementary for identifying both autism and developmental delay (DD). Parents completed autism and developmental screens before 18-month visits. Children with failed screens for autism (n = 167) and age, gender, and practice-matched children passing screens (n = 241) completed diagnostic evaluations for autism and developmental delay. When referral for autism and/or DD was considered, overall false positives from the autism screens were less frequent than for referral for autism alone. Presence of a failed communication subscale in the developmental screen was a red flag for autism and/or DD. An ordinally-scored autism screen had more favorable characteristics when considering autism and/or DD, yet none of the screens achieved recommended standards at 18 months, reinforcing the need for recurrent screening as autism emerges in early development.