K-RAS mutations are the most frequent molecular genetic alteration in serous ovarian tumors of borderline malignancy (SBOT). The pathogenesis of associated contralateral tumors and extraovarian implants and Müllerian inclusion cysts is obscure. We hypothesized that the comparison of K-RAS mutations in these lesions might help to distinguish multifocal from metastatic foci. Eight cases of SBOT with known K-RAS mutation (RAS+) and two cases without mutation (RAS-) were analyzed for comparison. DNA was extracted from multiple samples of 58 paraffin-embedded and laser-microdissected ovarian and extraovarian lesions (10 ovarian borderline tumors, 8 contralateral tumors, 25 implants, 15 inclusion cysts; total: 97 samples). K-RAS exon 1 was amplified by PCR and analyzed by denaturing gradient gel electrophoresis and cycle sequencing. In 12 of 14 SBOT and in 2 of 2 extraovarian implants, the K-RAS mutation could be found in different areas of the same lesion, indicating monoclonality. All RAS+ ovarian borderline tumors with contralateral tumors (six of six) harbored an identical mutation in both ovaries (in one case, a separate surface borderline tumor component contained a different mutation in addition). In 4 of 5 RAS+ ovarian tumors with extraovarian lesions, RAS mutations were also found in implants (15/21 implants [71%]) and more rarely in inclusion cysts (3 of 12 lesions [25%]). These extraovarian mutations were always identical to the one in the ovary (18 of 18 [100%]). Regarding the contralateral and extraovarian lesions of the two RAS- SBOT, only one extraovarian implant contained a RAS mutation. The demonstration of K-RAS mutations in Müllerian inclusion cysts and implants of SBOT suggests that K-RAS mutations represent a pivotal event during neoplastic transformation of ovarian and extraovarian serous epithelium. Considering our observations, the two putative pathogenetic mechanisms for the development of implants and endosalpingiosis-multifocal tumorigenesis and spread from the ovarian primary tumor-seem to coexist.