More than 26,000 new cases of ovarian cancer are identified each year in the United States, with almost 75% of these malignancies in advanced stages at the time of diagnosis. Early-stage disease has a cure rate of up to 90%, but the long-term survival rate of patients with advanced disease is 5-20%. At this time, there are no biomarkers that are effective indicators of early ovarian cancer. Recently, immunohistochemical and Southern blot studies have suggested that overexpression/amplification of the oncogene ERBB2 (HER2/neu) is associated with aggressive ovarian malignancies; however, some studies have not supported this conclusion. Because tumor cells are known to be highly heterogeneous, we used fluorescence in situ hybridization (FISH) to study individual ovarian cancer cells for HER2/neu amplification and chromosome 17 centromere copy number. Simultaneous multicolor cohybridization of HER2/neu and chromosome 17 centromere alpha-satellite probes were carried out on 43 ovarian cancer samples. Ten of the forty-three samples showed moderate to high amplification of HER2/neu, with varying numbers of chromosome 17 centromeres present. In some cells the amplified HER2/neu was dispersed throughout the nucleus, whereas in other cells the amplified oncogenes were clustered together. Within a sample there was heterogeneity in oncogene and centromere copy number. In this small study, we were unable to identify a specific clinical correlation. However, FISH is a powerful method for the study of oncogene amplification in tumor samples.