In order to determine the effectiveness of a Gastrointestinal Severity Index to screen for gastrointestinal disorders, the Gastrointestinal Severity Index was administered to 135 children with autism spectrum disorders and 146 comparisons with and without gastrointestinal disorders. The mean Gastrointestinal Severity Index scores of the groups were 3.53 ± 1.78, 3.15 ± 1.99, 0.81 ± 1.25, and 0.29 ± 0.76 (comparative pediatric patients with gastrointestinal disorder = autism spectrum disorder + gastrointestinal disorder > autism spectrum disorder-gastrointestinal disorder > comparative pediatric patients without gastrointestinal disorder, respectively), Ps < 0.05. Receiver operating characteristic curves and areas under the receiver operating characteristic curves were calculated to ascertain which Gastrointestinal Severity Index cutoff scores yielded the highest sensitivity and specificity rates for the diagnosis of gastrointestinal disorders. The area under the receiver operating characteristic curve (0.97) for the comparison group was higher (P < 0.001) than the area under the receiver operating characteristic curve (0.85) for autism spectrum disorder children indicating that the Gastrointestinal Severity Index was more effective in screening for gastrointestinal disorders in comparisons. However, the same Gastrointestinal Severity Index cutoff score of 2 and above yielded, respectively, sensitivity and specificity rates of 92% and 93% for comparisons and 80% and 79% for autism spectrum disorder children. The negative and positive predictive values based on these sensitivity and specificity rates were calculated for a range of prevalences of gastrointestinal disorders and indicated that the Gastrointestinal Severity Index may be useful for screening children with and without autism spectrum disorder for gastrointestinal symptoms.