The results of two nationwide administrations of the 1988 General Educational Development (GED) tests were compared. The first, a norming study, was based on 34,548 GED tests taken by graduating high school seniors. The second study was based on 55,154 GED test item sets administered to adult examinees who had taken the test for diploma equivalency. In general, the adults who passed the GED tests performed similarly to the graduating high school seniors on all GED tests. Both these groups significantly outperformed the GED failing group on each of the five multiple-choice tests. Observed results were similar for the essay test, but no significant differences were noted. Subtest analyses on the multiple-choice tests classified each by content area and cognitive level or item type. No significant differences were noted between seniors and adults who passed the GED. In most of these analyses, both the seniors and the adult pass group significantly outperformed the GEDT fail group. The results support use of the 1988 GED tests for awarding high school diploma equivalency credentials. Five tables and 12 figures are included. (SLD)