To better understand the biological and clinical features seen in the four major histologic types of ovarian carcinoma (OvCa), gene expression was measured in 113 primary OvCas using oligonucleotide microarrays, and the data were analyzed using principal component analysis. A representative view of the first three principal components is presented, using 158 histologic type-specific genes from the microarray data generated from 53 serous, 33 endometrioid, 10 mucinous, 8 clear cell, and 9 mixed histology OvCas. Type-specific genes were selected as those that were at least 2-fold increased in one histologic type compared to each of the others (considered separately) and had a t-test p-value <0.01. The representative H&E sections (insets) of each major histologic type of OvCa demonstrate the morphologic heterogeneity of the disease. Gene expression profiling most effectively distinguishes clear cell from other ovarian carcinomas. For details, see the article by Schwartz et al. on page 4722 of this issue.