Apolipoprotein E genotypes in 88 unrelated Japanese patients with NINCDS-ADRDA sporadic Alzheimer's disease (AD) were examined and compared with those of 93 healthy controls. Frequency of epsilon 4 allele was increased in patients with AD (31%) compared with controls (10%), as was reported previously. Individuals homozygous or heterozygous for the allele epsilon 4 had a 5.9-fold increased risk of AD. This tendency was more pronounced in early onset sporadic (= non-familial) type than late onset type. The relative risk was also greater for early onset type (RR = 11.7; 95% CI, 4.9-28.3) than late onset type (RR = 4.3; 95% CI, 2.1-8.8). Moreover, patients with homozygote for the allele epsilon 4 had a 14.7-fold increased risk of early onset sporadic AD (P < 0.005, chi 2 = 9.0, df = 1, 95% CI, 2.5-85.1). Our findings indicated that association of apolipoprotein epsilon 4 with sporadic Alzheimer's disease is more pronounced in early onset type than in late onset type.