Self-reported, dietary intake and biochemical estimates of thiamine, riboflavin, folate, vitamin B-12, protein, and iron were compared in 22, free-living elders by individuals who had senile dementia of the Alzheimer's type (SDAT) and in 41 who were cognitively normal (CN). The two groups did not differ significantly in their intake of these nutrients or the number of deficiency states for intake (less than 67% RDA). Low serum transketolase (thiamin; p less than 0.055), red blood cell (RBC) folate (p less than 0.06), and serum vitamin B-12 (p less than 0.05) levels occurred more often in SDAT patients than in CN subjects. Individuals in both groups who used multivitamin supplements had significantly higher biochemical values for thiamine (p less than 0.03), riboflavin (p less than 0.01), and vitamin B-12 (p less than 0.003) than nonsupplement users. Because of the differences in vitamin B-12 and RBC folate levels between groups, a retrospective analysis was performed on a larger group of subjects drawn from a geriatric assessment clinic. Patients with SDAT had significantly lower serum vitamin B-12 (p less than 0.01) and lower RBC folate (p less than 0.03) values than CN subjects. Which mean values for vitamin B-12 and RBC folate were grouped by degree of impairment in SDAT subjects, vitamin B-12 was significantly lower in mildly and moderately impaired subjects than in those with normal cognition. Mean values for both nutrients did not differ significantly between severely impaired and CN subjects. There was a significant quadratic relationship between cognitive impairment and biochemical values for vitamin B-12.(ABSTRACT TRUNCATED AT 250 WORDS)