The validity of the Stanford Binet-IV (SB-IV) was assessed. This test and the WAIS-R and WRAT-R were administered to 42 adults previously classified with mild to moderate mental retardation. Validity coefficients between scores on the SB-IV and the other two measures were significant. The mean IQ on the SB-IV (mean Test Composite = 43.26) was significantly lower than that on the Wechsler Adult Intelligence Scale-Revised–WAIS-R (mean Full-scale IQ = 57.91). With regard to the internal validity of the SB-IV, the intersubtest relationships of each of the four Area scores correlated significantly with the Test Composite (range = .66 to .91). Verbal Reasoning earned the highest correlation (.91). Results support the SB-IV's concurrent, criterion-related, and internal validity for use with young adults who have mental retardation.