An incidence survey of Hodgkin's disease in Greater Boston during 1959-1973 detected five sibling pairs under the age of 45. The expected number is 0.7; thus, siblings of young adults with Hodgkin's disease have about a sevenfold excess risk of the disease (P = 0.0008). Eight sibling pairs, not in the incidence series, were also identified. Among all 13 pairs, 12 were sex concordant; the number expected is 6.8 (P = 0.01). The literature includes 46 sibling pairs under 45 of which 30 are sex concordant. The expected number is 23.9 (P = 0.05). Combining the present and the literature series suggests that siblings of the same sex as an affected person have a risk of Hodgkin's disease double that of siblings of the opposite sex. The sex concordance suggests that the excess Hodgkin's disease among siblings of affected persons is due either to inter-personal transmission of an etiologic agent by prolonged or intimate contact or to common-source exposures.