The prevalence of cutis verticis gyrata was studied in a psychiatric institutional population of 494 patients, the majority of whom were mentally retarded or had chronic schizophrenia. Twenty-two subjects (21 males and 1 female) were found to have primary cutis verticis gyrata, yielding a prevalence of about 4.5%. The frequency of the scalp disorder was 11.4% among mentally retarded patients and 1.7% in schizophrenic subjects. A cytogenetic study was performed on patients with primary cutis verticis gyrata. In 9 out of 21 subjects there was evidence of chromosomal fragile sites: 5 patients had fragile sites on the X-chromosome, in 2 there was fragility of chromosome 12 and in 2, fragility of chromosome 9. The fragile X-site is the genetic marker of the 'fragile X-syndrome', a sex-linked inherited disorder often associated with mental retardation and other neuropathological findings.