A survey was carried out in northern Italy (Veneto and Friuli Venezia Giulia Regions) to assess the extension of canine heartworm disease close to the classic hyperendemic areas of the Po Valley. A filtration test on 275 privately owned dogs showed a mean prevalence of 44%, with a peak of 55% in the Venice province. A high prevalence (17%) of occult filariosis was discovered using a serological test. These values fall within the medium-high range previously reported from other Italian areas. The provinces of Pordenone, Udine, Trieste, Treviso and Belluno, where no cases had been previously reported, were found to be infected. The distribution of the infection did not differ significantly with the dogs' origin or sex. The prevalence seemed to be influenced significantly (p < 0.01) by the hosts' age, function and degree of outdoor activity, reflecting the different exposure risks.