Determining diagnostic stability of ASD, as well stability of functioning in early childhood, is relevant to prevalence, best practices for communicating early ASD diagnoses to caregivers, families’ experiences, and developmental trajectories. Generalizability of findings from prior research has been limited by small and homogenous samples, short follow-up time intervals, and inconsistent diagnostic procedures. This report presents follow-up evaluations of 60 children (86.7% male, mean age: 51.3 months) with diverse backgrounds (79.7% racial/ethnic minorities) who received initial ASD diagnoses before 36 months of age (mean age: 27 months). Fifty-three children (88.3%) met diagnostic criteria for ASD at follow-up, a proportion consistent with previous studies. On average, children demonstrated significant cognitive gains and ASD symptom improvement. Clinical implications of findings are discussed.