This study examined factors associated with 9-month outcomes of 78 preschoolers with autism spectrum disorder ([ASD]; 66 males, mean age = 44.8 [7.0] months) receiving preschool special education services. Use of recommended intervention practices was observed in each student’s primary educational setting: autism-only (n = 28), mixed-disability (n = 25), and regular early education (n = 25). Recommended practice use across setting types and developmental changes were compared using generalized estimating equations. Autism-only settings had the highest implementation of classroom structure, classroom environment, and curriculum and instruction. Regular early education had greater implementation of practices to support social and peer relationships. Supporting social and peer relationships was the largest predictor of gains, controlling for setting type; the association was greater for children with lower initial receptive language or developmental skills. These findings highlight the importance of supporting and promoting peer engagement in students with ASD across preschool placement types. (author abstract)