Elevated pretreatment serum interleukin-10 (IL-10) is associated with inferior progression-free survival (PFS) in patients with Hodgkin's disease (HD) treated with ABVD or equivalent regimens. Therefore, we explored the association of serum IL-10 with presenting features and PFS in HD patients treated only by radiotherapy (RT) with curative intent. Eligible patients were previously untreated, had biopsy-proven HD, were older than 16 years, HIV-negative, and had unthawed pretreatment serum. Serum IL-10 levels were measured with ELISA and were considered high if > or = 10 pg/ml. We identified 69 patients with median age of 34 years (range 16 - 74), of who 52% were males, and 3% had B-symptoms. Ann Arbor Stage was I in 35%, II in 58%, and III in 7% of the patients. Histology was lymphocyte predominance in 26%, and classical HD in 74% of the patients. Serum IL-10 was elevated in 35% of the patients. After a median follow-up of 67 months for survivors, the 5-year PFS of patients with high vs. normal serum IL-10 was 50% vs. 81% (all patients, P = 0.006), and 43% vs. 77% for the subset with classical HD (P = 0.008). Multivariate analysis revealed that high serum IL-10 and beta2-microglobulin were independently associated with inferior PFS. Patients with none, 1, or 2 adverse features comprised 57%, 36%, and 7% of the population, and their 5-year PFS was 80%, 63%, and 0%, respectively (P < 0.0001). In conclusion, high serum IL-10 is independently associated with inferior PFS in patients with HD treated with RT.