A clinical and cytogenetic study has been made of subjects from families who have possible X linked mental retardation. The families were distinguished as those with a clinical diagnosis of Renpenning syndrome and those with other behavioural or physical abnormalities obviating such a diagnosis. All subjects with REnpenning syndrome carried a fragile Xq27-28 chromosome in more than 4% of their blood lymphocytes. In addition, two other families who did not have Renpenning syndrome but had similar clinical features also carried the fragile site Xq27-28. A female age effect was observed and one possible carrier of Renpenning syndrome exhibited the fragile X in 10% of her lymphocytes but was also mentally retarded. Subjects within the same family did not always exhibit the fragile site on a comparable proportion of their cells.