The effect associated with the presence of seizures in 2 to 5-year-old autistic children was investigated in the largest and the longest observational study to-date. Parents assessed the development of 8461 children quarterly for three years on five orthogonal subscales: combinatorial receptive language, expressive language, sociability, sensory awareness, and health. Seizures were reported in 958 children (11%). In order to investigate the effect of seizures, children with seizures were matched to those with no seizures using propensity score based on age, gender, expressive language, receptive language, sociability, sensory awareness, and health at the 1st evaluation. The number of matched participants was 955 in each group. Children with no seizures developed faster compared to matched children with seizures in all subscales. On an annualized basis, participants with no seizures improved their receptive language 1.5-times faster than those with seizures; expressive language: 1.3-times faster; sociability: 2.3-times faster; sensory awareness: 6.2-times faster; and health: 20.0-times faster. This study confirms a high prevalence of seizures in ASD children and informs on the effect of seizures on children's longitudinal developmental trajectories.