The pathogenesis of Kaposi's sarcoma (KS) is still unclear, and several factors appear to be involved in the onset of the Kaposi's lesion. Epidemiological studies suggest that a common infective agent may contribute to KS. Sequences which appear to represent a new gammaherpesvirus, currently termed KSHV/HHV8, have recently been identified in KS. To further examine the relationship between this virus and KS, we obtained biopsy samples of KS lesions; these samples, the spindle cells cultured from these lesions and the PBMC of the same patients were tested for the presence of KSHV sequences by PCR. In addition, we tested several l ate passage KS spindle cell lines as well as control samples. The biopsy samples were from lesions of the following forms of KS: one sporadic KS, two epidemic KS and three iatrogenic KS, one of which was in the process of regressing after reduction of immunosuppressive therapy, and two that were at different stages (patch and nodular) from a single patient.The sporadic KS specimen was positive, as were the PBMCs from this patient, and cells grown from this biopsy appeared to contain KSHV viral sequences up to the fifth passage. Both epidemic KS biopsies were positive, but in these cases KSHV sequences were not detected in the cultured cells. The biopsy from the regressing iatrogenic KS lesion was negative, as were the cells cultured from this lesion. However, the PBMCs of this patient were weakly positive for KSHV at the time of biopsy, and PBMCs collected from this patient one month later were completely negative. The samples of both the patch and the nodular KS lesions obtained from another immunosuppressed patient showed amplifiable sequences of KSHV, but both the PBMCs of this patient and primary KS cell cultures from these biopsies were negative. Of the late-passage KS lines tested, only one, IST AIDS KS 12, was positive for KSHV. This line is derived from an early angiomatous-macula lesion. Taken together, these data suggest that an active KSHV infection is associated with KS and that elimination of KSHV from the lesion precedes regression of the lesion, strongly correlating KSHV with KS. In addition, early KS lesions may have a higher KSHV burden, or contain cells more susceptible to KSHV infection, further linking KSHV to KS.Afin d'analyser les relations entre le sarcome de Kaposi (KS) et le virus KSHV/HHV8, de la sousfamille des Gammaherpesvirinae, recemment identifie, nous avons etudie au moyen de la PCR les sequences KSHV presentes dans des biopsies de KS, dans les cellules fusiformes cultivees et dans les lymphocytes peripheriques de ces memes cas de KS (une biopsie d'un cas sporadique, deux de case pidemiques et trois de cas iatrogenes dont l'une chez un patient en regression apres reduction du traitement immunosuppresseur et les deux autres, a des stades differents, chez un autre malade). La biopsie et les lymphocytes peripheriques du cas sporadique sont positifs et les cellules cultivees a partir de cette biopsie ont des sequences virales KSHV jusqu'au 5 e passage. Les biopsies des deux cas epidemiques sont positives mais les sequences n'ont pas ete decelees dans les cellules cultivees. La biopsie du cas iatrogene regressif est negative, comme le sont les cellules cultivees a partir de cette lesion; mais les lymphocytes peripheriques sont faiblement positifs au moment de la biopsie et se montrent completement negatifs apres 1 mois. Les lesions de types differents analysees chez l'autre malade apres immunosuppression presentent des sequences amplifiables tandis que les lymphocytes peripheriques de ce malade et les cultures primaires de ces biopsies sont negatives. Parmi les lignees de passage tardif de KS etudiees, la lignee IST AIDS KS 12 s'est revelee positive pour le KSHV. Cette lignee est derivee d'une lesion angiomato-maculeuse precoce. L'ensemble de ces resultats suggere qu'une infection KSHV active est associee au KS, que l'elimination du KSHV des lesions precede leur regression et que les lesions precoces du KS peuvent avoir une charge virale elevee ou contenir des cellules tres sensibles a l'infection virale, le tout montrant l'etroite relation entre le KS et le KSHV.