In a longitudinal study, slowing of the EEG in SDAT patients and elderly controls is investigated using both visual and power spectral analyses. For this a reliability study in 20 SDAT patients is performed and results for intra-rater, inter-rater and test-retest reliability (in 24 h) are used in the selection of parameters. Next, the diagnostic efficacy of the chosen parameters for SDAT is confirmed in a validity study in 43 SDAT patients, 41 elderly non-organic psychiatric patients and 51 elderly normals. In the longitudinal study 78 of these subjects could be retested after 20 months. The parameter for visual analysis appeared to be superior in actually showing a progress of slowing of the EEG in 20/21 SDAT patients over time. The parameter for power spectral analysis was superior in measuring the more subtle processes in the normal elderly. Results of these studies are used to discuss some problems in the measurement of EEG in SDAT over time.