Recent natural catastrophes with large-scale loss of life have demonstrated the need for a new technique to provide information for disaster victim identification when DNA methods fail to yield the identification of an individual, or in other situations where authorities need to determine the recent geographical life history of people. The latter may be in relation to the identification of individuals detained on suspicion of terrorism or in relation to people-trafficking or smuggling. One proposed solution is the use of stable isotope profiling (SIP) using isotope ratio mass spectrometry (IRMS). Exploiting the link between the isotopic signal of dietary components and the isotopic composition of body tissue, the aim of this study was to refine a non-invasive method of analysing human material such as scalp hair and fingernails using SIP and to assess the degree of natural variability in these profiles. Scalp hair and fingernail samples were collected from British and non-British volunteers at Queen's University Belfast every 2 weeks for a minimum of 8 months. Samples were analysed using IRMS to determine their isotopic composition for 13C, 15N, 2H and 18O. The results of this longitudinal study yielded information on the natural variability of the isotopic composition of these tissues. The data demonstrate the relatively low degree of natural variation in the 13C/15N isotopic abundance of scalp hair and fingernails whilst greater variations were recorded in the hydrogen and oxygen values of the same samples. The 15N and 18O values of nail are noticeably more variable than that of scalp hair from the same subject. A hypothesis explaining this trend is put forward based on the faster rate of formation of hair than of nails. This means that there is less time for the compounds forming hair to be affected by biochemical processes that could alter their isotopic signature.