The purpose of this study was to determine the accuracy of reporting of invasive cancer by relatives for family studies. First, we attempted to evaluate whether a lower than expected cancer rate found in second-degree relatives of children with soft-tissue sarcoma was a result of underreporting. Second, we evaluated the accuracy of reported cancer in two data sets by comparing reported cancer information with documentation by medical records and death certificates. We obtained medical histories from a primary informant, usually the proband's parent, on 346 first- and 784 second-degree relatives of 68 childhood and adolescent soft-tissue sarcoma patients. To investigate underreporting by the primary informant we conducted an individual interview with each adult relative or proxy. Primary informants reported 22 cancers in first-degree relatives, all confirmed as invasive cancer, and 71 cancers in second-degree relatives with 50 of 67 for which documentation confirmed as invasive. Of 715 individual informants contacted, 15 additional cancers were reported, including 5 confirmed as invasive. The number of first-degree relatives with confirmed invasive cancers was within the expected range; however, the number of cancers in second-degree relatives was below the expected range (observed/expected = 0.51 (54/105.5) 95% confidence interval (CI) = 0.39-0.67). Thus, the lower than expected number of cancers in second-degree relatives was not attributable to underreporting by a single informant or inability to obtain documentation. The overreporting of 25 cancers (24.5%) in second-degree relatives, indicates the need to document all reported cancers.(ABSTRACT TRUNCATED AT 250 WORDS)