The levels of pterin compounds in the lumbar cerebrospinal fluid (CSF) of ten neurologically normal and twenty autistic children were measured by two different liquid chromatographic systems, a conventional and a newly developed direct method. Among pterin compounds 7,8-dihydroneopterin (NH2) and 6R-5,6,7,8-tetrahydrobiopterin (R-BH4) levels in autistic children were significantly reduced to 66.1 and 41.5%, respectively, of those found in the controls. The autistic children were divided into three subgroups according to their CSF homovanillic acid (HVA) level and age: the group with an elevated HVA level, the younger (< 7 year old) group with a normal HVA level, and the older (> 7 year old) group with a normal HVA level. In comparison, the further reduction was observed in NH2 and R-BH4 levels in the younger group and in the 7,8-dihydropterin (PH2) level in the group with an elevated HVA level. Thus, these results suggest that the endogenous biosynthesis of R-BH4 in the brain may be reduced in autistic children as compared with that in neurologically normal children.