Ovarian cancers from 64 midwestern US women were screened for p53 dysfunction both by immunohistochemical staining (IHCS) and single strand conformation polymorphism (SSCP) analysis of the entire open reading frame (ORF). Forty SSCP abnormalities in 39 tumors included nine deletion, one insertion, two splice junction, two nonsense, one silent and 25 missense mutations were confirmed by direct genomic sequencing. Eight of the insertion/deletion defects may have occurred due to slippage during the course of DNA replication. This observation suggests that genomic instability may play an important role in ovarian carcinogenesis. Fifteen percent of the mutations encountered were located outside exons 5-9 and four of these were null. The sensitivity of IHCS was 96% for missense mutations but only 14% for null mutations. This contrasted with 100% sensitivity of the SSCP screening methodology. The 21% overall incidence of null mutations in the present study far exceeds the reported 6.8% incidence in the world literature (P=0.0003). Explanations for this difference include: (1) our complete analysis of the entire ORF of the p53 gene; (2) the tendency of others to rely upon IHCS to screen tumors prior to mutation analysis; and (3) environmental or endogenous genetic influences.