Patients with a positive reaction to the Paul-Bunnell test in the period 1961-72 were identified at nine different laboratories in Norway and matched against cases of malignant lymphoma registered at the Cancer Registry of Norway in the period 1961-75. Among 5,840 patients having a positive Paul-Bunnell test a total of six developed malignant lymphoma, three of these more than 1 year after the positive Paul-Bunnell test. The expected number of malignant lymphoma was 2.04. Of the six lymphoma cases, five were classified as Hodgkin's disease (HD). The present investigation agrees well with the findings of other epidemiological studies. There seems to be a small excess in incidence of HD among patients with previous infectious mononucleosis (IM), but both diagnostic problems and possible confounding factors must be taken into account before a possible causal association is considered.