About one fourth of patients with Hodgkin's disease relapse after therapy. The mechanisms that lead to resistance to treatment in these patients are poorly understood. The authors describe the differential protein expression of p53, proliferating cell nuclear antigen (PCNA), and p21 at initial presentation and relapse, and discuss their role in disease progression and resistance to therapy. Thirty-four patients with Hodgkin's disease who had relapsed after standard chemotherapy and radiotherapy regimens were assessed for the expression of p53 protein, PCNA, and p21 protein (waf/cip 1). In 14 of these cases, sequential biopsies performed both at presentation and at relapse were available for the study. Seventy-five percent of the cases were positive for the p53 protein. Tumors at relapse had higher p53 and PCNA scores than those at initial presentation. In the paired samples, a significant increase was noted in the number of p53 and PCNA-positive cells and in the intensity of staining with p53 antibody. Six of seven paired samples tested for p21 showed an increased p21 expression at relapse. These results suggest that, at relapse, Reed-Sternberg (RS) cells and their variants positive for p53, PCNA, and p21 are increased in number and individually have an increased expression of p53, PCNA, and p21 proteins. These findings suggest that therapy failure and relapse may, at least in part, be associated with altered p53, p21, and PCNA pathways. HUM PATHOL 28:549-555. This work was carried out during an exchange fellowship program at the National Cancer Institute, Bethesda. There are no restrictions on its use