In 69 patients on chronic haemodialysis, blood sampled randomly during dialysis was analyzed for carboxyhaemoglobin (COHb). The median value was 1.40% (range 0.9-2.3) in non-smoking patients and (1.4-7.5) in smokers. In non-smokers treated with erythropoietin (EPO) correlation was found between COHb and the weekly EPO dose (r = 0.57, p = 0.007). In smoking patients not given EPO, the COHb correlated well with the number of cigarettes smoked (r = 0.84, p = 0.003). The COHb values did not correlate to the haemoglobin values. It is concluded that COHb levels in uraemic non-smokers are elevated because of increased endogenous CO production from the enhanced erythrocyte turnover. As even low COHb levels may negatively influence the oxygen status of the uraemic patient, the addition of exogenous CO from cigarette smoking should be avoided.