For more than a decade it has been convention to assume that senile dementia Alzheimer's type (SDAT) and Alzheimer's disease early onset represent a unitary disease process with only an onset difference. This assumption has been neither confirmed nor disconfirmed. To address this issue, a study was conducted which analyzed the dissolution of thought in SDAT and in Alzheimer's disease early onset. Subjects were 25 elderly adults with SDAT, 14 adults with Alzheimer's disease early onset, and 25 normal elderly adults who provided a measurement baseline. The SDAT group had an age range of 65-91 years, the normal elderly had an age range of 65-95 years, and the early onset adults had an age range of 44-60 years. The measures for level of thought processing were based on Piaget's universal, unilinear, hierarchical, structural stages of development of thought. Subjects were administered five tests for concrete operational thought. Analysis of quantitative and qualitative data suggests that, although Alzheimer's disease early onset is similar to SDAT in its fundamental pattern of dissolution of thought, the early onset syndrome is dissimilar in that it is characterized by an acceleration of the rate of dissolution found in SDAT. (NB)