A study was carried out to assess the field efficacy of ivermectin (IVM) and pyrantel pamoate (PYR) against Parascaris equorum. Seventy-three foals (3-18 months old) from 5 stud farms, not treated with anthelmintics in the previous 10 weeks and with individual faecal egg counts (FEC) >200, were included in the study. For each stud farm, 5-7 foals were included in the IVM-treatment group (IVM 0.2%, 200 mcg/kg body weight) or in the PYR-treatment group (PYR 38%, 13.2mg/kg body weight) and 3 were untreated as controls. For each foal, FECs were carried out before treatment (Day 0) and on Days 7 and 21. An individually based estimation of efficacy was assessed by a bootstrap simulation applied to different previously suggested formulae to evaluate the percent reduction of faecal egg counts (FECR). Two thousand bootstrap resamples were constructed from individual FECRs and the parasite population was considered susceptible for FECs >or=90 and 95% confidence interval (C.I.) >or=95%, suspected resistant for FECRs comprised between 80 and 90% and 95% C.I. <95% and resistant when FECR <or=80 and 95% C.I. <or=90%. IVM treatment was effective in foals from 2 farms, was equivocal at both study times in 1 farm and ineffective in 2 farms, while PYR was fully effective on all farms. This data shows that P. equorum-resistant populations are present also in Italy and the phenomenon is spreading in horses throughout Europe.