The Se status of 177 Greek cancer patients has been assessed by fluorimetric analysis of the Se concentration in whole blood, urine (morning) and hair. Patients either with newly diagnosed cancers or with metastases or who are undergoing therapy have statistically significant less blood, urine and hair Se than age- and sex-matched healthy controls. There is a strong and significant correlation between blood and urine or hair Se in the newly diagnosed cancers. In cancers of systems involving absorption, metabolism and excretion of Se, and in female breast cancer, the Se levels in blood, urine and hair are significantly lower than in the controls. In cancers of the respiratory and haematologic systems, blood and urine Se are normal, while hair Se is significantly lower than in controls. The results are compared with literature data; conflicting data may be due to different dietary habits of the various populations. Some cancerous tissues, such as those of the peptic system, accumulate Se compared with their adjacent, apparently healthy tissues, but others, of the urinary system for example, do not. The reason(s) for this behaviour, however, is not known.