Remarkable differences were observed when 1,465 healthy Caucasian individuals and 128 healthy Negro individuals were compared for the genetic distribution of 25 different HLA antigens. Caucasians had a significantly higher frequency of A1, A3, B8, and Bw16, and Negroes of A28 and Aw30. The haplotype which had the highest incidence as well as the greatest positive linkage disequilibrium was A1-B8 among Caucasians and A2-B12 among Negroes. Genetic distance between the two races was 0.0592. The 89 Caucasian patients with renal failure did not demonstrate any significant deviations in phenotype frequencies (PF) of various antigens, when compared with healthy Caucasians; however, 48 similar Negro patients had twice as high an incidence of Bw17 as the healthy Negroes. No significant deviation in PF was observed in 77 Caucasian patients who had leukemia; however, 32 Caucasian patients who had back pain (24 also had back stiffness) due to spondylitic, arthritic or disc syndrome, had a significant increase of Bw16 and of B27, and a decrease of B12, when compared to Caucasian controls.