A well-researched report from a working group of 23 experts from 13 countries, convened to evaluate the evidence of carcinogenicity of drinking-water disinfectants and 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).