A total of 124 households with gas water heaters comprising a total of 241 individuals were interviewed particularly for symptoms of carbon monoxide poisoning. The 124 gas water heaters were investigated technically and the production of carbon monoxide was measured. Twenty (16%) of the gas water heaters investigated produced carbon monoxide in quantities of more than 0.1 volume % and these are regarded as potentially dangerous. Significant difference in the symptom score average was found for the group with carbon monoxide concentrations of less than 0.1 volume % compared with carbon monoxide concentrations greater than or equal to 2 volumes % (p<0.05). The frequencies of syncope and seizures, headache and poor concentration were found to be significantly different in the corresponding groups (p<0.05).