Radiotherapy is known to cause secondary malignancies in the radiation field; postradiation sarcomas (PRS) are one example of such malignancies. Little is known about the genetic changes, including p53 gene alterations, that are thought to play a role in the tumorigenesis of human PRS. In the present study, p53 gene mutations were analyzed on paraffin-embedded specimens from 24 patients with PRS (4 men and 20 women) by polymerase chain reaction-single-strand conformation polymorphism (PCR-SSCP) followed by direct sequencing. The primary tumors of these patients were uterine cervical cancers in 14, breast cancers in 3, malignant lymphomas in 2, and others in 5. Total radiation doses ranged from 36 to 300 Gy (median, 60 Gy). The latent period between completion of radiation therapy and development of PRS ranged from 3 to 34 years (median, 10 years). Malignant fibrous histiocytoma was the most common PRS, accounting for 12 cases. PCR-SSCP revealed the aberrant mobility shifts of bands in 24 cases: 21 shifts in exon 5, 18 in exon 7, and 12 in exon 8. Direct sequencing of the SSCP product revealed a total of 58 mutations in 21 (88%) of 24 cases: 4 cases had a single mutation, 5 had 2 mutations, 5 had 3 mutations, 6 had 4 mutations, and 1 had 5 mutations. Although 31% of the mutations did not change an amino acid, every tumor had at least one mutation that did, which may have provided the selection pressure for expansion. The frequency of p53 gene mutation in sporadic soft tissue sarcomas was 20%. These findings highlighted the extraordinarily high frequency of p53 gene mutations in PRS. G:C to A:T transition at dipyrimidine sites was found in 14 (58%) of 24 cases. Collectively, these findings indicate that radiation is causative for soft tissue sarcomas via p53 gene mutations.