Two cases of Laurence-Moon-Biedl syndrome were reported with marked mental symptoms. Case 1 had mental defficiency, obesity, polydactylia and Retinitis pigmentosa, and was recognized periodic mental symptoms (repeated alternatively increased psychomotor activity and decreased psychomotor activity). Case 2 had mental defficiency, obesity, polydactylia and Retinitis pigmentosa, and was recognized constant mental symptoms (lacking in spontaneity, mutism, negativism and so on). The mental symptoms of Laurence-Moon-Biedl syndrome seems to be classified as two groups. That is, the first group has constant mental symptoms and the second group has periodic mental symptoms. Our Case 1 belongs to the second group and Case 2 belongs to the first group.