In this study, we used oxygen‐ and hydrogen‐isotope data from human bone (δ18O) and modern environmental water samples (δ18O and δD) to investigate geographic origins of individuals buried at Cahuachi, a ceremonial centre in the Nasca region of Peru (c.AD1‐1000). Our objective was to characterise the natural variation in water stable isotopic composition in the Rio Grande de Nasca drainage, and then to use these data to better infer place of origin for 30 adults interred at Cahuachi. Using the δ18O and δD values of 63 modern environmental water samples, it was possible to differentiate among the northern and southern river middle valleys, and to infer the isotopic composition of drinking water at higher elevations. Over half of the individuals included in this study had drinking water oxygen‐isotope compositions consistent with places of origin away from Cahuachi during the last 10 to 25 years of life, perhaps in the northern river middle valleys or in the upper valleys/sierra. The environmental water stable isotopic baseline developed in this study enabled a better understanding of the natural variation of waters in the Rio Grande de Nasca drainage. As a result, it was possible to assess the geographic range of place of origin for these individuals with greater certainty. Taken together, these data support the idea of Cahuachi as a place of both local and regional significance, with individuals from distant parts of the Rio Grande de Nasca drainage travelling to and/or transporting the dead to the site for death or burial. Copyright © 2011 John Wiley & Sons, Ltd.