Previous studies have reported a drop of IQ in males with Martin-Bell or fragile X syndrome during childhood and adolescence. It is uncertain when and why this drop occurs and whether it affects all fragile X males. We have analyzed longitudinal IQ data on all 24 fragile X males who have been followed for at least 2 years through the Child Development Unit in Denver and who have been evaluated with a Stanford-Binet Form L-M assessment. A significant drop in IQ occurred for the group as a whole, but an individual z analysis demonstrated that only 7 of 24 had a significant decline in IQ. Visual inspection demonstrates an initial decline in IQ beginning in middle childhood and continuing through adolescence. We postulate that this drop occurs because of relatively greater weaknesses with abstract reasoning and higher symbolic language skills that are stressed in the cognitive testing of later childhood and adolescence.