Progressive cognitive impairment is a defining feature of the dementia of Alzheimer's disease (AD), yet disagreement exists over which abilities decline most precipitously and which cognitive tests are more sensitive. In this study, 51 AD patients in the early to middle stages of illness and 22 age-matched normal controls were administered a battery of neuropsychological tests at 6-month intervals over a 2-year period. While the performance of the normal controls remained stable over the 2 years, the AD patients displayed progressive decline on all tests. The greatest declines occurred on tests requiring lexical/semantic processing (Boston Naming Test) and comprehension of syntactic relationships (Token Test). Performance on visuospatial tests (Wechsler Adult Intelligence Scale-Revised Block Design, Benton Visual Retention Test, Spatial Delayed Recognition Span Test) declined less rapidly. The findings support previous reports that language impairment may be central to the dementia of AD, and that confrontation naming is particularly sensitive to decline in this illness.