The measurement of soluble transferrin receptor (sTfR) has been proposed as a novel approach to the diagnosis of iron deficiency, especially in anaemia of chronic diseases (ACD). Our aim was to study the utility of sTfR under 'everyday conditions' as seen in a geriatric hospital in the following groups of patients: First, in a pilot group of 99 multimorbid geriatric patients (85 women, 14 men; 82.00 +/- 6.32 years) admitted for rehabilitation after recent surgical treatment of a bone fracture; second, in 677 geriatric patients (506 women, 171 men; 79.17 +/- 11.47 years) with different diagnoses admitted to a department of internal medicine; third, in some remarkable clinical cases in order to illustrate the diagnostic limits of sTfR. In general, both genders showed a remarkable age-dependent decrease in erythropoiesis. In patients with haemoglobin levels below 12.0 mg/dL, this parameter correlated significantly with sTfR. However, this was seen only in women, not in men. Moreover, an age-dependent increase in sTfR was seen in women, while in men it remained almost constant. Based on these findings, we conclude that there is a different, gender-specific aetiology of iron deficiency in the elderly. About 30% of patients of both genders simultaneously had low haemoglobin levels and low sTfR. This was interpreted as 'adaptation' or 'tolerance' to the iron deficiency. This was illustrated by a clinical case of megaloblastic anaemia: Initially low sTfR rose only during the vitamin B12 substitution and normalized after recovery. We conclude that sTfR provides an insight into the 'dynamics' of iron metabolism: A rise in sTfR indicates an 'acute readiness to refill iron stores', while a low (non-stimulated) sTfR level corresponds to the quite frequent adaptation to iron deficiency and/or inhibition of resorption. Finally, extremely high sTfR levels were observed in some cases of malignancy such as in acute leukaemia and in hypernephroma. Thus, increased sTfR levels can be caused by paraneoplastic effects.