There are an estimated three million hired migrant and seasonal farmworkers in the United States. Adults and children may be exposed to mutagenic and potentially carcinogenic pesticides during planting, weeding, thinning, and harvesting crops. Field conditions that provide little opportunity to wash skin or clothing to minimize pesticide absorption may intensify exposure. Little is known, however, about the occurrence of cancer in migrant or seasonal farmworkers. Most cancer epidemiologic research on agricultural populations has focussed on farm owner/operators. The few studies that have evaluated cancer in farmworkers suggest that, like farm owner/operators, they may be experiencing excesses of multiple myeloma and cancers of the stomach, prostate, and testis. A few studies suggest that the farmworkers may differ from farmers by experiencing excesses of cancers of the buccal cavity and pharynx, lung, and liver. Cervical cancer was elevated in female farmworkers in one study. Descriptive data and etiologic research on cancer among farmworkers and family members are urgently needed. Feasibility evaluations, however, should precede etiologic investigations because of possible difficulties in studying this population of workers. Issues that need to be evaluated include assessing where and when farmworkers and family members are diagnosed and/or treated for malignancies, the ability of farmworkers to provide histories of crops, locations, and years worked and living conditions, the ability of agricultural experts to determine likely pesticide exposures based on such farmworkers' histories, the ability to obtain information on potential confounding factors, the ability to recontact or determine vital status of specific farmworkers over time, the suitability of conducting studies in home-base vs. upstream counties, and the ability to study agriculturally related malignancies in persons who have left farm work before the disease occurs.