We have reinvestigated 33 fragile X families using probe StB12.3. In 31 families the affected individual showed an insert while in 2 families no insert was detected. The insert fell into two size categories: small (less than 0.5 kb); and large (greater than 0.6 kb) accompanied by methylation of an EagI site. All individuals of either sex having a small insert were fra(X) negative and intellectually normal, while all males having a large insert were fra(X) positive and intellectually impaired. Females having a large insert were either fra(X) positive or negative and either intellectually normal or impaired. No new mutation was found. All daughters of males with a small insert had a small insert; females with a large insert produced males and females who had a large insert, while females with a small insert had offspring with either a large or a small insert. However, females with a small insert tended to fall into one of two categories: either they had only children with a small insert or only children with a large insert, there being only one exception to this rule. We found four unexpected small inserts, two in unrelated spouses and two in female carriers who proved to be compound heterozygotes, indicating that they had inherited an insert from both their parents. These observations suggest that individuals with a small insert must be not uncommon in the general population.