The NGHS I was initiated in 1985 as a contract-supported program with three field centers and a coordinating center. Annual exams were conducted at baseline and through the years of follow-up. The study was renewed by means of the cooperative agreement mechanism in 1992 as the NGHS II to permit two additional years of follow-up exams. The study was again renewed in 1994 as NGHS III to continue annual examinations.