The headwaters of the Río Pilcomayo drain the Cerro Rico de Potosí precious metal-polymetallic tin deposits of southern Bolivia. Mining of these deposits has taken place for around 500 years, leading to severe contamination of the Pilcomayo's waters and sediments for at least 200 km downstream. Communities living downstream of the mines and processing mills rely on the river water for irrigation, washing and occasionally, cooking and drinking, although most communities take their drinking water from springs located in the mountains above their village. This investigation focuses on arsenic exposure in people living in riverside communities up to 150 km downstream of the source. Sampling took place in April–May 2003 (dry season) and was repeated in January–March 2004 (wet season) in five communities: El Molino, Tasapampa, Tuero Chico, Sotomayor and Cota. Cota was the control in 2003 and again in 2004; a nearby city, Sucre, and several locations in the UK were also used as controls in 2004. Drinking, irrigation and river waters, hair and urine samples were collected in each community, digested where appropriate and analysed for As using ICP-MS. Arsenic concentrations in drinking waters ranged 0.2–112 μg 1–1, irrigation water 0.6–329 μg 1–1, river waters 0.9–12,800 μg 1–1, hair 37–2110 μg kg–1 and urine 11–891 μg 1–1. All but one drinking water sample was found to contain As below the World Health Organization recommended guideline of 10 μg 1–1, although a number of irrigation and river water concentrations were above Canadian and Bolivian guidelines. Many As concentrations in the hair and urine samples from this study exceeded published values for non-occupationally exposed subjects. Analysis of mean concentration values for all media types showed that there were no statistically significant differences between the control locations and the communities exposed to known As contamination, suggesting that the source of As may not be mining-related. Arsenic concentration appears to increase as a function of age in hair samples from males and females older than 30 years. Male volunteers over the age of 35 showed increasing urine-As concentrations as a function of age, whereas the opposite was true for the females.