The cerebrospinal fluid (CSF) levels of neuropeptide substance P were measured in 20 patients with Rett syndrome and controls at Kurume University, Japan, and other centers. CSF substance P levels are constant in control children between 2 and 12 years, and show a gradual decrease through adolescence, reaching a plateau at 20 years. Significant reductions in substance P in patients with RS compared to controls were present at the early phases of the disease, at age 2 to 3 years, and were not age dependent. Childhood RS levels were 50% of controls in the same age group, and adults with RS had 37% of control adult values. The mean CSF levels of substance P in patients with mental retardation, epilepsy, and Guillain-Barre disease were not different from controls without neurologic disease. 
COMMENT. CSF concentrations of substance P have been shown to reflect brain and spinal cord concentrations. The authors suggest that decreased CSF levels of substance P in patients with RS, if confirmed in other studies, may be useful as a biological marker for the disease.