Purpose <p>Intelligence testing is an important tool for clinicians to help in diagnosis and treatment planning for children with neurodevelopmental disabilities. Although interpreting IQ profiles is a common clinical practice, there are concerns regarding the validity and reliability of such scores. Previous research has shown relatively stable global measures of IQ, but less stable scores within index or subtest score. More research is needed to better understand the long-term stability of IQ scores within autistic and other neurodevelopmental populations who may have more instability in their scores over time.</p> Methods <p>We tested the long-term stability of IQ scores using the same test over time (Stanford-Binet, Fifth Edition; SB-5) in a large clinical sample of 650 youth with autism (<i>n</i> = 236) or other neurodevelopmental disabilities (<i>n</i> = 414), over a period of 4 months to 11 years. In addition to IQ scores, we also tested consistency of scatter scores, overall profiles, and strengths and weaknesses using linear mixed effects models.</p> Results <p>Results indicated overall consistency of FSIQ scores (ICC = 0.86), slightly less stability for VIQ and NVIQ, and low stability for abbreviated IQ as well as index and subtest scores. The consistency of cognitive profiles, scatter scores, and strengths and weaknesses was poor. Younger age was the best predictor of higher FSIQ instability.</p> Conclusion <p>Long-term stability of IQ in neurodevelopmental disabilities appears similar to results from other studies with different clinical and nonclinical groups. Implications and recommendations are discussed.</p>