The incidence of Acanthamoeba keratitis has been assessed in the 1995 West of Scotland Keratitis Study. All patients with microbial keratitis in the 3 million population were reported to us prospectively by optometrists, ophthalmologists and microbiologists. Contact lens (CL) wearing patients completed detailed questionnaires; 50 healthy CL wearers were visited at home as controls and similarly investigated. Additionally all 340 optician practices were polled for their annual turnover of CL types and disinfectant solutions.Acanthamoeba keratitis occurred with an incidence of 110000 CL wearers based on this large, reliable community study of 164 000 CL wearers. This is 25 times more frequent than the previous American estimate (i.e. 1:250 000). The relationship to CL material type, ineffective cold chemicals solutions and hygiene compliance is complex, but is directly related to use of contaminated tap water as part of CL care, this vehicle being the source of the amoebae.These findings are of considerable concern. They suggest that many new cases of painful, distressing and potentially blinding infection will be seen in Asia as CL wear becomes more widely available. Local expertise for recognition of Acanthamoeba keratitis will be required plus treatment with a combination of 0.02% chlorhexidine and 0.1% propamidine. Tap water must be avoided in CL care, and rigorous attention paid to CL hygiene.