Advances continue in erythropoietin biology, and additional data reviewed here have recently become available on complex feedback mechanisms describing the interrelations of hypoxia and its effects on anemia and tumor behavior (eg, apoptosis, angiogenesis). In addition to biology, other clinically relevant data in oncology are included and an attempt is made to identify patients who are most likely to benefit from treatment. The latter aspects will better define the profile of the target patient, probably prevent overtreatment, and improve cost-benefit ratios. Interesting data on radiotherapy results improved by increasing tissue hemoglobin have been published but will need further confirmation.