The relationship between selected aspects of medical history and the risk of non-Hodgkin's lymphomas (NHLs) was investigated using data from a hospital-based case-control study conducted in northern Italy on 177 cases of NHL and 561 controls in hospital for acute conditions, other than nonneoplastic or immunological. Among six viral diseases considered, only herpes zoster (shingles) had a relative risk (RR) significantly above unity [RR = 2.7; 95% confidence intervals (CI), 1.5 to 4.7]. The association, however, was restricted to subjects whose diagnosis of herpes zoster dated back to less than 10 years, suggesting that this slow-acting virus could be reactivated by the early development of NHL. Six of eight bacterial diseases considered showed RR above unity, and the estimate was significant for scarlet fever (RR = 1.9; 95% CI, 1.1 to 3.5) and pyelonephritis (RR = 5.3; 95% CI, 1.8 to 16.2). These associations were not restricted to the few years before lymphoma diagnosis. When various classes of infectious or inflammatory diseases were grouped together, no association was evident for viral infections (RR = 0.8; 95% CI, 0.6 to 1.2), acute bacterial diseases (RR = 1.0; 95% CI, 0.7 to 1.5), or allergic conditions (RR = 1.0; 95% CI, 0.6 to 2.1). The risk estimates were nonsignificantly above unity for chronic bacterial diseases (RR = 1.2; 95% CI, 0.7 to 1.2) and autoimmune conditions (RR = 1.4; 95% CI, 0.9 to 2.2), and significantly elevated for chronic inflammatory disease (RR = 1.9; 95% CI, 1.2 to 3.0).(ABSTRACT TRUNCATED AT 250 WORDS)