Using the single-strand conformational polymorphism technique, we have screened 66 malignant ovarian tumors for p53 mutation in exons 5 to 8. Thirty-four of the tumors demonstrated a single-strand conformational polymorphism band shift in this region of the gene, including 6 in exon 5, 7 in exon 6, 12 in exon 7, and 10 in exon 8 (one of the tumors showed a shift for exons 7 and 8). All of the single-strand conformational polymorphism shifts have been further characterized by DNA sequencing, and 31 of 35 have been shown to represent genuine DNA alterations. These include 27 point mutations (23 missense, 2 nonsense, and 2 silent mutations), 3 deletions (a 2-base pair deletion introducing, by frameshift, a stop codon further downstream; a 3-base pair deletion; and an unusual 6-base pair deletion made up of separate 2-base pair and 4-base pair deletions), and a 4-base pair insertion (introducing a stop codon downstream). In total, 29 of the 66 (44%) carcinomas analyzed had mutations affecting the primary sequence of the p53 protein. p53 mutation was found in tumors of all International Federation of Gynecologists and Obstetricians stages, suggesting that it might be an earlier genetic event in the progression of epithelial ovarian tumors than previously thought. A significantly greater number of p53 mutations were seen in high-grade serous carcinomas than in those of endometrioid and mucinous types (0.02 > P > 0.01). Analysis of the distribution of point mutations showed no preference for any particular mutation type.