We followed up 67 children with autistic disorder (AD) and 31 children with pervasive developmental disorder not otherwise specified (PDDNOS) for more than 10 years by reviewing medical records at a clinic for children with developmental disabilities. The participants' data were collected between their first visit to the clinic and the visit at which they applied for basic disability benefits. The standardized IQ scores and autistic symptoms were examined as measures of the children's personal functioning. For environmental factors, we examined the participants' educational placements and work and residential status. Using structural equation modeling, we examined the longitudinal developmental courses of AD and PDDNOS. Participants diagnosed with AD consistently showed lower IQ and more severe autistic symptoms than those diagnosed with PDDNOS. Relationships between personal functioning and environmental factors differed between the two groups. AD and PDDNOS are heterogeneous, so they must be treated differently to improve children's prognoses.