The association of Non-Hodgkin Lymphoma (NHL) and HIV-infection was soon recognized and the Center of Disease Control (CDC) has classified some types of NHL as AIDS-defining illnesses (ADI). Hodgkin's disease (HD) represents the most common type of non ADI malignancy in HIV-infected cases. Commonly, data on malignant lymphoma in this population is collected in known HIV-positive patients or in autopsy-series. This registration study was designed to estimate the incidence of HIV-positivity in patients with newly diagnosed malignant lymphoma. A registration of all patients with newly diagnosed malignant lymphoma and their HIV-status was performed in every center of the Swiss Group for Clinical Cancer research (SAKK) from January 1, 1991 to July 31, 1993. Among 474 eligible patients, HIV-status was evaluated in 400 and 52 were tested positive (13%), 42 (81%) of them males. Three of them were newly detected cases (after lymphoma-diagnosis). Three hundred and forty patients (72%) presented with NHL, 42 (12.4%) of them HIV-positive; 33 out of these had aggressive lymphoma. B-symptoms were significantly more frequent in HIV-positive patients. In the 134 patients with HD, 10 (7.5%) tested HIV-positive, mostly presenting with stage IV disease (7), B-symptoms (9) and extranodal disease (7). In conclusion, 13% out of 400 evaluated patients with newly diagnosed malignant lymphoma tested HIV-positive. The study confirms the predominance of aggressive lymphoma histologies and frequent presentation with B-symptoms in HIV-positive patients with NHL. Male gender, young age (26-35 years) and B-symptoms are prognostic factors for HIV-positivity in NHL.