PURPOSE: To compare the changes with increasing age of ERG parameters in relation to clinical data in two distinct phenotypes of genetically determined, dominantly inherited macular drusen: malattia leventinese (ML) and Zermatt macular dystrophy (ZMD). METHODS: Ganzfeld rod- and cone-electroretinograms (ERGs) from 15 patients affected with ML and 14 patients with ZMD and clinical data were analyzed retrospectively. The patients' ages ranged from 20 to 77 years in the ML group and from 9 to 74 years in the ZMD group. RESULTS: Both inherited macular degenerations caused a marked decrease in visual acuity, the latest after age 65. Most patients with ML retained good visual function (0.8-1.0) until the fifth decade, followed by a rapid decrease in the fifth or sixth decade. ZMD is characterized by a relatively continuous decrease in visual acuity with increasing age. Morphologically, in the juvenile stages in both entities, drusen were observed at the posterior pole. Rod-driven and cone-driven ERG b-wave amplitudes decreased nearly linearly in ML and ZMD in accord with the normal loss of amplitude with increasing age. Implicit times of cone b-waves for ML increased markedly with age, whereas in ZMD the values were always prolonged beyond the normal range with a slight increase with age. CONCLUSIONS: In terms of visual acuity, the progression of both dominantly inherited macular dystrophies is quite different. This is not reflected in the amplitudes of the b-waves in the Ganzfeld ERGs, which decrease normally for both entities. Implicit times of the cone-b waves were more markedly prolonged in ML compared with ZMD. In-depth longitudinal documentation of the natural course of those dominantly inherited macular diseases should facilitate patient counseling.