Serum beta‐2‐microglobulin (S‐β2M) was measured at diagnosis in 44 patients with lymphocytic leukemias and 47 with malignant lymphomas. Among patients with chronic lymphocytic leukemia (CLL) S‐β2M was raised (>3 mg/1) in 74% and in 23.5% of those with acute lymphoblastic leukemia (ALL). The frequencies for non‐Hodgkin's lymphoma (NHL) and Hodgkin's disease (HD) were 59.2% and 40%, respectively. In CLL patients high serum values correlated with large tumor mass, as estimated by Rai's clinical criteria (P < 0.001), by total peripheral lymphocytes (r = 0.41, P < 0.05) and by the percentage of bone marrow infiltration of the lymphocytes (P < 0.01). A significant relation was also found in CLL patients between S‐β2M level and survival (P < 0.05). In ALL no association was found between S‐β2M level with peripheral lymphoblast concentration, French‐American‐British (FAB) subclassification, splenomegaly, and survival. In NHL patients a significant association was found between S‐β2M levels and stage of disease (P < 0.01) and an obscure relation (P < 0.01) and an obscure relation (P < 0.1) with the presence of lymph nodes greater than 3 cm in diameter, splenomegaly, and hepatomegaly. No significant association was found between S‐β2M level and histologic subtypes, presence of B symptoms, bone marrow involvement, and survival. In HD patients a significant association was found between the level of S‐β2M and stage of disease (P < 0.05) and presence of splenomegaly (P < 0.05). No association was found between S‐β2M level and histologic subtypes, lymph nodes greater than 3 cm in diameter, bone marrow involvement, and B symptoms. A significant relation was found between S‐β2M level and survival in HD patients with widespread disease (P < .025).