The concentration of serum IgD was measured in 100 patients with Hodgkin disease; 65 had 3--50-fold increases in IgD levels. Several parameters were found to influence serum IgD concentration: IgD level in older patients (greater than 50 yr) was significantly lower than in the younger patients (P less than 18 yr). IgD concentration was significantly higher in splenectomized than in nonsplenectomized patients (p less than 0.005). Therapy was found to depress IgD concentration, which fell to a value significantly lower than in untreated patients (p less than 0.05). An interesting correlation was found between IgD levels and histologic type of the disease, lower levels being preferentially associated with the lymphocyte predominance type and higher with the lymphocyte dipletion type. The logarithms of the means of these two groups were significantly different from the overall mean of the disease (p less than 0.05 and p less than 0.0005, respectively). Mixed cellularity and nodular sclerosing showed intermediate values.