As in many human malignancies, TP53 mutations are the most common genetic alterations in malignant human ovarian tumours. An approach often used in the determination of TP53 status is immunohistochemical staining of the protein. Non-missense mutations, especially those of the null type, causing premature termination codons and resulting in truncated proteins, may often not be detectable by immunohistochemistry. Therefore, current estimates of TP53 alterations in ovarian cancer may be inaccurate. By using polymerase chain reaction-single strand conformation polymorphism analysis and sequencing techniques, we have found a high prevalence of TP53 non-missense mutations in exons 5-8 in ovarian tumour specimens from patients from the southwestern part of The Netherlands. Twenty-nine of 64 tumours showed mutations, of which 10 were non-missense mutations. The majority (9 of 10) of these non-missense mutations, including 7 nonsense mutations and 2 frameshift deletions, were null type mutations and could not be detected by immunohistochemical staining. Five of the 7 nonsense mutations were mutations at codon 213 (Arg8Stop). The nature of the high prevalence of this nonsense mutation in our series of ovarian carcinomas remains unknown. In addition to the 9 null type mutations, a splice junction mutation was encountered. In conclusion, we have observed a high prevalence (13%) of ovarian tumours with null type mutations in exons 5-8 that did not result in immunostaining. Our data suggest that, especially in ovarian cancer, immunological assessment of TP53 is not an adequate tool to study TP53 alteration. A frequent nonsense mutation at codon 213 in 5 (8%) of 64 tumour specimens represents an important finding.