In a case-control study of 280 out of 426 consecutive patients with a recent diagnosis of non-Hodgkin lymphoma (NHL) and 1827 control subjects, 53 (19%) and 230 (13%) respectively had received blood transfusions 1 year or more before the interview. Using an age- and sex-stratified analysis the odds ratio (OR) for transfusion was 1.74 (95% CI 1.24-2.44). ORs were also determined for transfusions received in the intervals 1-5, 6-15, 16-25 and > or = 26 years before diagnosis. In the interval 6-15 years, the OR for transfusion was 2.83 (95% CI 1.60-4.99) whereas ORs for transfusions received in other intervals were lower and not significantly elevated. Histological diagnoses (Kiel classification) and results of staging procedures were known for 185 patients. For low-grade NHL of nodal B-cell chronic lymphocytic leukaemia (B-CLL) or immunocytoma type, the OR for transfusions was 4.15 (95% CI 1.92-9.01). For low-grade nodal lymphomas of follicle centre cell type and high-grade nodal lymphomas, no relation to transfusions could be demonstrated. For high-grade extranodal lymphoma as sole manifestation, OR for transfusions was 3.27 (95% CI 1.30-8.24). It is concluded that blood transfusion may be a risk factor for NHLs especially those of B-CLL or immunocytoma type and for high-grade extranodal lymphoma.