BACKGROUND C-reactive protein (CRP), an acute phase reactant, plays an important part in the investigation of the role of inflammation in pathology. Many cardiovascular events show significant associations with various cosmophysical parameters. The aim of this study was to investigate the relationship between the level of CRP and the levels of solar, geomagnetic (GMA), and cosmic ray activity. METHODS The results of 25,399 serum CRP tests carried out over a 3-year period were compared with the monthly and daily levels of solar, geomagnetic, and cosmic ray activity during the same period. The cosmophysical data were obtained from the National Oceanic Atmospheric Administration (NOAA) in the U.S. and from the Russian Academy of Science. RESULTS On a monthly basis, CRP levels showed no correlation with GMA (n=36, r=0.258, p=0.13), but there was a significant inverse relationship with neutron activity (r=-0.35, p=0.03). For the daily comparisons, CRP levels were significantly correlated with GMA (n=1057, r=0.97, p=0.02), and there was a significant inverse relationship with neutron activity (r=-0.97, p=0.025). Daily neutron activity was higher on days with CRP levels of 0-1.0 mg/dl (n=289) and above1.0-1.5 mg/dl (n=1213) than on days with higher CRP values (>1.5; n=23,897; p<0.0001). CONCLUSION The level of serum CRP, in addition to its association with inflammation, is related to the daily level of GMA and inversely to the level of neutron activity.