Characteristics of intraneuronal lipopigment in two siblings with Sanfilippo's syndrome are reported. A lipopigment emission spectrum probably reflects its composition and the (uncorrected) autofluorescence emission spectra results are compared with spectra from non-diseased tissue and from previously reported childhood-onset neuronal ceroidlipofuscinoses (ceroidoses), adult-onset ceroidosis (Kufs' disease) and animal ceroidoses. Values derived from the emission spectra from Sanfilippo's syndrome could be distinguished from those obtained from equivalent regions of non-diseased brains and were within the range of abnormal values previously reported from accumulations of pigment in various types of neuronal ceroidosis. Some abnormal lipopigment in Sanfilippo tissue was indistinguishable from some lipopigment in childhood-onset ceroidosis and in Kufs' disease. These results indicate that the intraneuronal lipopigment which accumulates to an abnormal extent in Sanfilippo's syndrome should not be termed "lipofuscin", which is a normal cerebral constituent, but "ceroid" to denote lipopigment with abnormal characteristics.