To determine whether the age of onset of Alzheimer's disease (AD) is related to the expression and rate of decline of this disorder, we examined the clinical and neuropsychological performance of 421 patients entered into the Consortium to Establish a Registry for Alzheimer's Disease and followed annually for up to 4 years. Statistical analyses were based on multivariable logistic regression analysis for dichotomous clinical measures and multivariable linear regression analysis for psychometric measures. All analyses examined the effect of age after controlling for gender, education, and stage of dementia. Clinical information obtained on entry into the study indicated that younger patients performed more poorly on measures of language and concentration, and older patients performed more poorly on measures of memory and orientation. On neuropsychological measures at entry, younger patients performed more poorly on praxis and had significantly higher scores on confrontation naming. Younger age predicted a significantly faster rate of progression for all neuropsychological measures. These findings support the concept of age-related clinical subtypes of AD.