As the flow of pollen-carrying transgenes is considered to be one of the main problems associated with the release of genetically modified plants, the impact on the environment of transgenic tomato plants highly resistant to cucumber mosaic virus (CMV) was evaluated. The frequency of spontaneous crossing between homozygous transgenic tomato, cv. UC82B, and untransformed controls was assessed in two Italian growing areas. Transformed plants expressed the CMV coat protein (CP), inducing resistance to the virus, and neomycin phosphotransferase II (NPTII), used as an in vitro selectable marker. For each field, a sample of at least 2000 progeny plants of the `wild-type' UC82B control was screened to search for `hybrids' expressing the transgenic proteins. Kanamycin sprayed on seedling leaves and DAS-ELISA were used to detect NPTII and CMV-CP, respectively. Plants expressing the transgenic proteins were then analysed by PCR using CMV-CP gene-specific primers. Two plants from each field were found to express the transgenes. These plants, however, proved to be homozygous for transgenes, indicating that they did not originate by cross-pollination between transgenic and `wild-type' parents, but probably derived from transgenic seeds that accidentally contaminated untransformed seed stocks during harvesting. These results show that under our conditions, no transgene flow was detected in cv. UC82B tomatoes in the two fields.