Pesticides are chemicals designed to cause biologic harm. Wide spread use in agriculture and in urban areas creates an opportunity for exposure to a large segment of the population. The carcinogenicity of some pesticides in bioassays raises concerns about human exposure and underscores the need to know which may pose a human cancer risk. Previous studies of male farmers have linked lymphatic and hematopoietic cancers with several pesticides. Results from case-control studies of multiple myeloma and non-Hodgkin's lymphoma in Nebraska indicate that women farmers may experience similar risks. Excesses among women were more strongly linked to use of pesticides on animals than on crops. Risks associated with pesticides were especially high among subjects with first degree relatives with cancer. Analysis of pooled data from three case-control studies of non-Hodgkin's lymphoma among men showed no association with use of the herbicide atrazine. Ongoing cohort studies of lawn care workers, herbicide applicators, and aerial applicators are designed to evaluate cancer risks among pesticide applicators with unique exposure situations. Blood and urine have been collected from a sample of applicators applying the herbicide 2,4-D to relate dose to possible biologic effects including DNA mutations, chromosomal aberrations, and immune system alterations. Data from case-control studies of brain and stomach cancer recently completed on men and women farmers from Nebraska are being analyzed to identify agricultural exposures that might explain the high rates for these tumors seen among farmers in many developed countries. The Agricultural Health Study is a major effort designed to evaluate the relationship between agricultural factors and cancer and other diseases. Enrollment for this prospective study will be completed by the end of 1996 and will include approximately 75,000 farmers, pesticide applicators, and farmers' wives. Detailed information is being obtained by interview on pesticides and other agricultural exposures, lifestyle factors, medical and family history of disease and diet. The cohort will be linked to the National Death Index to obtain information on cause of death and to cancer registries to obtain information on cancer incidence.