C-reactive protein (CRP) belongs to Pentaxin family of proteins synthesized in the liver. It is the main acute phase protein, because its level is a very sensitive indicator of the host response to infection, inflammatory reactions and injuries ongoing with tissue necrosis. The ”CRP high sensitivity test” which was introduced some years ago – enables very precise determination of this protein concentration, differing only slightly from the normal concentration. Several applications of the test were described in this paper: to differentiate type I diabetes with vascular, to recognize two stages in post heart infarct period, and in rheumatoid arthritis (RA). In RA, it was useful in retrospective evaluation of the disease onset and in precise monitoring of the treatment. In our own investigations we showed the diagnostic advantage of the test in patients with cerebral stroke with infectious complications.