To obtain information on possible sources of contamination of the seed harvest of oilseed rape (Brassica napus L., spp. napus) by other varieties (adventitious presence), we investigated the purity of certified seed lots; the abundance and origin of volunteers; and longevity and origin of seeds in the soil seed-bank. This information was acquired through DNA analysis of volunteers collected in the field and seedlings derived from the soil seed-bank. DNA profiles of the volunteers and seedlings were obtained using Inter Simple Sequence Repeat (ISSR) markers, and the profiles were compared with ISSR profiles from an assortment of 14 of the most commonly cultivated oilseed rape varieties from 1985 to 2004. This comparison was performed using the assignment program, AFLPOP. The age of the seed bank germinating to become volunteers was assumed from information on previously cultivated oilseed rape varieties at the site and the year of cultivation. The results showed that plants or seeds belonged to varieties cultivated at the site 4–17 years earlier, suggesting a long persistence of volunteer populations. High volunteer frequencies (6%, 29% and 32%) were found in the three oilseed rape fields analysed and, from soil cores, their soil seed-bank was estimated to be 50–100 seeds m− 2. The ISSR analysis of the 14 reference varieties showed that three of the certified seed lots contained other varieties above the allowed threshold. Our findings are discussed in the context of the coexistence between GM (genetically modified) and non-GM oilseed rape, and future levels of seed purity.