Carbon steel pipes lined with polyvinyl chloride (PVC) and polyethylene (PE) have been used for water supply and hot water recirculating systems in buildings over the past 20 years in Japan. Severe localized corrosion has been experienced at the pipe-end protection fittings of water supply piping. Case histories of the corrosion failures, efforts of improving the fitting structure, and the protection measures for rehabilitation for existing systems are described in the paper. Keywords: carbon steel pipe, PVC lining, polyethylene lining, soft water, water supply system, fittings, rehabilitation