Some children with autism spectrum disorders (ASDs) exhibit low mental age (Low-MA; i.e., cognitive functioning below 12 months). We examined diagnosis, symptom severity, and development in children with ASD-low MA (n = 25), autistic disorder (n = 111), and PDD-NOS (n = 82) at ages two and four. We predicted that some ASD-low MA children would demonstrate just intellectual impairment and not autism symptoms on follow-up, with social deficits at age two attributable to global delays. Instead, most ASD-low MA children (96%) had an ASD at follow-up, compared to children initially diagnosed with autistic disorder (86.5%) or PDD-NOS (73.2%). They showed the least developmental progress and highest symptom severity. Results support diagnosing ASDs in children functioning below a 12-month level.