The C-reactive protein, Chlamydia-specific IgG antibody, and fibrinogen were assayed in the serum of 159 patients with arterial disease (the arterial group) and 203 patients with heart valve prostheses (the valvular group) and no demonstrable coronary disease. In the arterial group, the Chlamydia pneumoniae antibody was greater than or equal to1 :32 for 67.3% (107/159) of the patients, the C-reactive protein was elevated in 41.5% (66/159), and the fibrinogen was elevated in 27.7% (44/159). In the valvular group, the C. pneumoniae antibody was greater than or equal to1 :32 for 59.1% (120/203) of the patients; the C-reactive protein was elevated in 34.0% (69/203), and the fibrinogen was elevated in 17.2% (35/203). Of 107 patients in the arterial group with C. pneumoniae titers greater than or equal to 1 : 32, only 26 (24.3%) had elevated fibrinogen (426 +/- 29 mg/dL) and 44 (41.1%) had elevated C-reactive protein (1.06 +/- 0.52 mg/dL). Similarly, of the 120 patients in the valvular group with C. pneumoniae titers less than or equal to 1 : 32, 17 (14.2%) had elevated fibrinogen (409 +/- 29 mg/dL) and 34 had elevated C-reactive protein (0.99 +/- 1.1 mg/dL). Correlated poorly was C. pneumoniae with C-reactive protein and fibrinogen levels. Only the fibrinogen level could be discriminated between the arterial and the valvular group. These results suggest that no causal association exists between inflammation and C. pneumoniae. A highly significant correlation between C-reactive protein and fibrinogen levels was found.