Leaf roll is, at present, the most important virus problem in connection with the production of certified seed potato in São Paulo, Brazil. Leaf roll virus determinations based on samples obtained from seed lots can offer a high degree of accuracy, but generally demand a period of time that interferes with the seed potato sales to growers. The use of potato stems, collected at random late in the season, for grafting tests to determine the occurrence of the leaf roll virus in the planting and consequently in the tubers, is considered very promising for the conditions encountered in São Paulo and feasible even on a large scale. The best results were obtained using 40-day-old Datura stramonium seedlings as a roptstock for top grafts with 60-70 days-old potato stems to be tested. Positive symptons can be recognized between 30 to 40 days after grafting.