Associations have been found between birth weight and many diseases in adult life. In most countries, few birth records exist for older adults; therefore, birth weight is usually obtained by maternal recall or self-report. This study examined determinants of the availability and accuracy of self-report in middle-aged and elderly women. Birth weights, recorded at the time of birth, were found in 1999 for a subset of 363 women participating in a long-running cancer research study in the United Kingdom. Questionnaires were sent to the surviving 286 women requesting information on their birth weight and other factors related to their birth family. Twenty-five percent of the 244 respondents were able to report their birth weight to within 4 ounces (113.4 g) of that listed in birth records, 28% reported it inaccurately, and 47% did not know their birth weight. The most important factors determining the availability of self-reported birth weight were having a living mother and a low birth weight (< or = 6 pounds (2,722 g)). The most important determinants of accuracy, for those who provided a report, were being younger and the eldest child. Research studies relying on self-reported birth weight should take these factors into account.