Sixty new cases of human dirofilariasis due to Dirofilaria repens, occurring in Italy between 1990 and 1999, are presented. This is the most extensive case study of this zoonosis reported worldwide by a single study group. The aim is to utilize this large experience to characterize the different histopathological findings in the parasitic lesions in man. Diagnosis was performed on histological sections of the nematode enclosed in the nodules excised at biopsy or surgery. The nematode was located in the subcutaneous tissue (49 cases), the epididymis (two cases), the spermatic cord (two cases), the lung (two cases), the breast (two cases), the omentum (two cases) and under the conjunctival tissue (one case). The majority of cases (46) were from Piedmont; the remainder were from Emilia-Romagna, Sardinia, Sicily, Tuscany, Apulia and Lombardy. The histopathological features of the lesions are described and the clinical and epidemiological aspects of the zoonosis are discussed. The prevalence in Italy in general and in the area of Piedmont in particular, comprising the provinces of Alessandria, Asti, Novara and Vercelli, which is one of the most severely affected areas of the world, is emphasized. The evident increase in the number of cases in the last few years is a clear indication that it is an emergent zoonosis. We recommend that each and every case observed be recorded, to enable the true extent of human dirofilariasis in Italy to be assessed, and that a reference centre be set up in the area to collate the data. The importance of the histopathologist's role in the diagnosis is stressed.