Time-spare "clusters" of leukemia cases have been reported since the 1930s; since 1960, a great number of such observations has been made. However, it is still an open question whether or not the phenomenon is real.5 "Clustering" of Hodgkin's disease cases is a relatively new phenomenon.%1i118 T h e reports appear more convincing than those relating to leukemia, but closer analysis reveals possible serious methodological flaws which make their interpretation difficult. In this paper we consider the currently more interesting Hodgkin's disease situation, using the leukemia studies as a guide as to what pitfalls should be avoided. We then reverse the process a n d suggest cluster studies that should now be conducted in leukemia o n the basis of what may b e learned from the Hodgkin's disease studies.
Cancer 34:1390-1394, 1974.
RIOR TO 1971, EPIDEMIOLOGIC EVIDENCE P pointing to Hodgkin's disease being infectious was very limited. T h e literature contained numerous reports of cases occurring in the same family, mostly sib-sib pairs, but these reports are virtually impossible to interpret. The best detailed study was conducted by Razis and his colleagues.ll They reviewed case notes at Memorial Hospital, NY, over the period 1918-1958, for evidence of Hodgkin's disease in the immediate family members of cases. They took as "controls" cases of leukemia, lymphosarcoma, other malignant tumors, and benign conditions, and also looked for Hodgkin's disease cases among their immediate family members. There was about a three-fold excess of Hodgkin's disease cases in the families of Hodgkin's disease patients compared with the controls. Further analysis of the familial cases showed that, in sib-sib pairs, the dates of onset tended to be