A working group of 23 experts from 13 countries met in Lyon to evaluate the evidence for carcinogenicity of arsenic (mostly naturally occurring) as a contaminant of drinking-water, and of the water-disinfectant chloramine. The working group also evaluated or re-evaluated four chlorination by-products found in drinking-water, namely chloral hydrate, di- and trichloroacetic acids, and 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (also known as MX). High-level exposure to arsenic in drinking-water occurs in some regions such as China, Latin America, Bangladesh and West Bengal. The Working Group reviewed epidemiological studies of human cancer (mainly ecological studies in Taiwan and Chile, and several case-control and cohort studies) in relation to arsenic in drinking-water. Arsenic in drinking-water (primarily inorganic, as arsenate and to a lesser extent arsenite) was evaluated as carcinogenic to humans (Group 1) on the basis of sufficient evidence for an increased risk for cancer of the urinary bladder and lung. Studies on inorganic arsenic in experimental animals provided limited evidence for its carcinogenicity, but sufficient evidence was found in experimental animals for the carcinogenicity of dimethylarsinic acid (an organic form of arsenic), which produced urinary bladder tumours in rats and lung tumours in mice after oral administration.