A method of estimating the total tumour burden in patients with Hodgkin's disease was devised, combining the number of involved regions with the tumour size in each region. Further, a method of estimating the total tumour cell burden was devised, combining the estimate of the total macroscopic tumour burden with an estimate of the concentration of tumour cells in the tumour tissue. The prognostic significance of the total tumour burden was examined in multivariate studies of 300 patients in pathological stages I and II treated in the Danish National Hodgkin Study and 506 patients in all stages treated at the Finsen Institute, Copenhagen, Denmark, during a 15-year period. The total tumour burden turned out to be the most important prognostic factor in Hodgkin's disease. Most of the hitherto known prognostic factors were shown to be correlated with the total tumour burden and to lack independent prognostic significance.