Abstract: This mini-review gives a brief account of the emergence of the electron paramagnetic resonance (EPR) spectroscopy in the second half of the 20^th century and reports the continuous wave EPR spectroscopy studies on human and animal blood. The question posed by this review is whether the EPR spectroscopy in the form it appeared 70 years ago is still able to provide useful information about different pathological conditions in humans, particularly in the area of diagnosis.