The relationship between various diseases and immunisations and the risk of multiple myeloma was analysed using data from a hospital-based case-control study conducted in Northern Italy on 117 patients with multiple myeloma and 477 controls. Associations were observed for clinical history of scarlet fever (relative risk, RR = 2.0; 95% confidence interval, CI = 1.1-3.9), tuberculosis (RR = 2.3%; 95% CI = 0.9-5.7) and BCG immunisation (RR = 3.0; 95% CI = 1.4-6.4). The relative risk was 1.8 (95% CI = 0.9-3.5) for episodes of Herpes zoster infection, but most of the excess cases occurred within 10 years of diagnosis, suggesting that this might have been an early manifestation of the disease. No association emerged for common childhood viral infections or any other immunisation practice. When various classes of infectious or inflammatory diseases were grouped together according to their aetiology, there was a significant positive association with chronic bacterial illnesses (RR = 1.8; 95% CI = 1.1-2.8), and the relative risk estimates increased with the number of bacterial diseases. The trend in risk with number of diseases was significant (chi 21 = 4.5, P = 0.03). A negative association was found between allergic conditions and risk of multiple myeloma (RR = 0.6; 95% CI = 0.3-1.0).