Objective: To investigate quantitative aberrations involving p53 copy numbers in eutopic endometrial and endometriotic tissue from two populations. Design: Comparative analysis of normal and diseased tissue. Setting: Tissue specimens collected in Iceland and USA. Patient(s): Subjects with moderate/severe endometriosis (Iceland, n = 26; USA, n = 45). Paraffin-embedded tissue from 19 matched Icelandic cases and seven unaffected controls. American cases were fresh surgical tissue from 17 matched cases and 28 unaffected controls. DNA isolation and real-time polymerase chain reaction (PCR) with TaqMan assay were performed. Main Outcome Measure(s): The frequency of p53 loss and/or gain based on quantitative differences for copy numbers of p53 located on chromosome (17p) and GAPDH on a control locus (chromosome 12p). Result(s): Among American cases, significant p53 gain (n = 13) or loss (n = 4) was observed in 17 of 21 cases. In Icelandic cases this was not seen to the same degree. Mean normalized p53 values were 3.46 and 1.16 copies per reaction, respectively. Significant differences were observed between normalized p53 in the control blood and affected tissue for the American and Icelandic cases compared to standard GAPDH control but not in normal Icelandic and American endometrium. Conclusion(s): The results continue to support a role for nonrandom somatic p53 locus alterations in the pathogenesis of late or severe-stage endometriosis. Differences between Icelandic and American subjects have implications for generalization of genome-wide approaches. Objective: To investigate quantitative aberrations involving p53 copy numbers in eutopic endometrial and endometriotic tissue from two populations. Design: Comparative analysis of normal and diseased tissues. Setting: Tissue specimens collected in Iceland and USA. Endometriosis (Iceland, n = 26; USA, n = 45). Paraffin-embedded tissue from 19 matched Icelandic cases and seven unaffected controls. American cases were fresh surgical tissue from 17 matched cases and 28 unaffected controls. DNA isolation and real-time Main Outcome Measure (s): The frequency of p53 loss and / or gain based on quantitative differences for copy numbers of p53 located on chromosome (17p) and GAPDH on a control locus ( Result (s): Among American cases, significant p53 gain (n = 13) or loss (n = 4) was observed in 17 of 21 cases. In Icelandic cases this was not seen to the same degree. Mean norma Significant differences were observed between normalized p53 in the control blood and affected tissue for the American and Icelandic cases compared to standard GAPDH control but not in normal Icelandic and American endometrium. Conclusion (s ) The results continue to support a role for nonrandom somatic p53 locus alterations in the pathogenesis of late or severe-stage endometriosis. Differences between Icelandic and American subjects have implications for generalization of genome-wide approaches.