The present study examines the reported condition and clinical treatment of children at the Psychological Clinic that Lightner Witmer (1867-1956) founded at the University of Pennsylvania for delinquent and "backward" children whose problems were considered to have been caused not by "feeble-mindedness", but rather by environmental and physical factors. It was thought possible for children who received continuous treatment there to improve their mental and physical abilities up to the typical level for their age. At the Clinic, individual treatments were designed based on continuing diagnosis conducted through a prolonged period of educational treatment; coordination with the children's homes and specialized agencies was also emphasized. An experimental special class organized by the Psychological Clinic and held during the summer vacations of public schools not only provided continuous individual treatment to children who had earlier visited the Psychological Clinic and were considered to need more detailed examination, but also practiced an experimental group treatment. The experimental special class was, however, never exclusively designed to help "backward" children who were thought to be capable of achieving the grade level and social status of children with typical development, due to the lack of methods for classifying "backwardness" and "feeble-mindedness". Although Witmer never developed systematic methods of diagnosis and treatment for these children as a group, he achieved some progress in diagnosing and treating individual cases. The Psychological Clinic was the world's first psychological clinic.