We developed and evaluated a new parent report instrument—Parent Observation of Early Markers Scale (POEMS)—to monitor the behavioral development of infants at risk for autism spectrum disorder (ASD) because they have older affected siblings. Parents of 108 at-risk infants (74 males, 34 females) completed the POEMS from child age 1–24 months. The POEMS had acceptable psychometric properties and promising predictive validity. Most concerning items were social and communication deficits, and intolerance to waiting. Results provide preliminary evidence that prospective parent report measures can help to detect early ASD symptoms in infants at biological risk. We invite researchers to join us in multi-center studies of the POEMS.