Watercress (Lepidium sativum) is consumed as a vegetable, especially by the indigenous community in New Zealand. An investigation was carried out on the accumulation of arsenic by watercress, following earlier reports of inordinate arsenic concentrations in some aquatic macrophytes collected from the Waikato River, North Island, New Zealand. The Waikato River and some other aquatic systems in Taupo Volcanic Zone, New Zealand have elevated arsenic concentrations due to geothermal activity. Watercress, river water and sediment samples were collected from 27 sites along the Waikato river and analysed for arsenic. Greenhouse trials with watercress grown in beakers containing added arsenic were conducted to confirm the ability of this species to accumulate arsenic. At a number of sites, the concentration of arsenic in both the water and the watercress samples exceeded the World Health Organisation (WHO) limit for drinking water (0.01 mg l(-1)) and foodstuffs (2 mg kg(-1) on a fresh weight basis). The average leaf and stem arsenic concentrations were, respectively, 29.0 and 15.9 mg kg(-1) on a fresh weight basis. Plants grown in solutions of >0.4 mg l(-1) arsenic concentration had fresh weight arsenic concentrations above the WHO limit. Despite these higher concentrations, arsenic levels in plants grown under greenhouse conditions were approximately fivefold lower than in plants growing in the Waikato River, possibly because under natural conditions, the watercress is rooted in sediment containing on average approximately 35 mg kg(-1) arsenic. It is recommended that watercress from the Waikato River, or other areas with elevated water arsenic concentrations, should not be consumed.