The HL-A phenotypes of 127 patients with Hodgkin's disease have been determined. A very significant association has been found between Hodgkin's disease and two HL-A antigens, HL-A11 (P < 0.009), and W5 (P < < 0.0005). The families of 40 of these patients were genotyped for HL-A antigens. A normal mendelian segregation of the relevant antigen was found in all 12 families of HL-All positive patients and in 6 of 8 families of W5 positive patients. These findings suggest that certain Hodgkin's patients have a genetically determined susceptibility to their disease. It is postulated that this susceptibility could be due to linkage between HL-A genes and genes controlling immune responsiveness. Analysis of subgroups of Hodgkin's patients based on age, sex, and pathology suggests that these HL-A associations are most marked in certain subgroups.