In the past, pigs were commonly fed with acorns, and this was of remarkable economic importance. Currently this habit is continued in some areas, especially for production of prime-quality Iberian ham. Mature acorns, after shedding and during storage in unsuitable conditions, can be quickly infected with spores of many moulds, which cause mummification, blackening, dehydration, and nutrient loss. This study aimed to evaluate the quality of acorns of pedunculate oak (Quercus robur L.). The samples were collected in 2017 in southern Wielkopolska (central part of Poland), as feed material. In mouldy acorns a very high number of fungi was found (2.6 × 106 cfu/g), and 97% of them represented pathogenic Penicillium spp. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis showed in mouldy acorns high concentrations of mycophenolic acid (14580 μg/kg) and patulin (50 μg/kg). The dominant mould species, Penicillium expansum, showed a high cytotoxicity of swine kidney cells using assay based on the conversion of the tetrazolium salt, 3-(4,5, dimethylthiazol-2-yl)-2-5 diphenyltetrazolium (MTT). This raises the question if the pathogenic metabolites of moulds present in acorns can be dangerous for livestock, especially pigs, and people, as acorns are beginning to be seen as an interesting and functional part of their diet.