Limonene has demonstrated e$cacy in pre-clinical models of breast and colon cancers. The purpose of the study is (1) to determine D-limonene levels in a variety of commonly consumed citrus juices and (2) to develop a citrus questionnaire (CQ) and a D-limonene database which can be used in epidemiological studies to assess the relationship between dietary D-limonene and cancer risk. Focus groups and semi-structured interviews were conducted to develop the questionnaire and to assess the consumption and preparation techniques of citrus foods. Seventy samples of various citrus juice products were analysed by gas chromatography. Preliminary results suggest that sources and preparation or storage practices have major in#uences on D-limonene levels. Many commonly consumed citrus beverages contain no D-limonene. The largest sources of D-limonene (mg/L), identi"ed to date, are fresh Mediterranean-style lemonade (1027) and limeade (402), followed by commercial orange juices in cans (51.9}85), in glasses (35.4}60.3), from concentrates (27.5}90) and grapefruit juices in cans (13.5}50). The availability of quantitative estimates of D-limonene will substantially enhance the scope of epidemiological studies by (1) allowing for the examination of the e!ects of dietary D-limonene and (2) minimizing the potential for uncontrolled confounding by unmeasured sources of D-limonene.