Seventy‐four sporadic ovarian tumors were studied for loss of heterozygosity (LOH) and microsatellite instability (MI) with 20 polymorphic markers on chromosome 17 and at least I marker on every other chromosome. Additionally, activation of the K‐ras oncogene was examined through mutation analysis of codon 12. A majority of the tumors analyzed were low grade and/or of the mucinous histologic type. A negative correlation between LOH on chromosome 17 and K‐ras activation was observed, with the former alteration present in the majority of high grade serous and endometrioid tumors and the latter most commonly found in the mucinous and low malignant potential (LMP) tumors. In 60% of cases where LOH on chromosome 17 was present, it was observed at all informative markers, indicating chromosome loss. In these cases, frequent events of LOH were observed on the other chromosomes. When confined events of LOH were observed on chromosome 17, fewer events of LOH were observed on the other chromosomes. In the absence of LOH on chromosome 17, LOH on other chromosomes was rare. K‐ras activation was most commonly observed in tumors with no LOH events. Two endometrioid tumors and 2 mucinous tumors demonstrated MI. Our data support the involvement of different molecular pathways in the development of different types of ovarian tumors. © 1995 Wiley‐Liss, Inc.