In a population of 432 apparently healthy men aged 50 years and over 19-2% had one or more erythrocytes per high-power field in the urine while 8-1% had more than 10. Erythrocyturia seemed to be strongly correlated with tobacco consumption. No significant association with occupation could be demonstrated but the data were insufficient to refute the possibility of such an association. Raised levels of orthoaminophenols as a result of abnormal tryptophan metabolism induced by smoking might cause microhaematuria in smokers. As these metabolites are carcinogens microhaematuria might be a valuable sign of cancer of the urinary bladder.