The annual incidence of non-Hodgkin's lymphomas is increasing by 3 to 4% in different parts of the developed world, while rates for Hodgkin's disease, myelomas and leukemias are more stable. In the case of this group of malignancies, hypothesis generation on risk factors has been limited by the use of the ICD classification in mortality and incidence statistics. We have computed incidence rates in different Italian areas after careful re-classification of diagnoses, and considering specific histotypes (Working Formulation for NHL, Rye's classification for HD). While no particularly interesting pattern is suggested for Hodgkin's disease (even after considering specific Rye subgroups), multiple myeloma and leukemias, for non-Hodgkin's lymphomas the high rate in one agricultural area (Forli) was mainly due to the A sub-group in the Working Formulation (low-grade). In a heavily industrialized area (Varese), the high incidence rate was at least partly explained by a higher proportion of cases classified in the G sub-group (intermediate grade). Excesses of non-Hodgkin's lymphomas have been observed in populations exposed to phenoxy-acetic-acid herbicides, to insecticides and to organic solvents. One can hypothesize that different risk factors act on different stem cells and induce lymphoid malignancies belonging to different histologic sub-types.