This study concerns platelet activity at myocardial infarctions and possible relationships with Chlamydia pneumoniae seroreactivity. Fourteen patients with acute myocardial infarction and ST-segment elevations were enrolled. They all received thrombolytic therapy. The subjects were examined within 24 h after hospital admission (Day 1) and after 6 months of recovery. On Day 1, C. pneumoniae IgM antibody titres were analysed and on Day 1 and during recovery C. pneumoniae IgG and soluble P-selectin were determined. P-selectin was used to estimate platelet activation. C. pneumoniae IgM titres at the infarction were closely related to both Day 1 IgG titres (r = 0.6; p < 0.05) and to IgG levels after 6 months (r = 0.8; p < 0.01). These results indicate a possible reactivation of a chronic infection. C. pneumoniae IgM was related to platelet activation. The correlation coefficient was r = 0.7 (p < 0.01) when comparing IgM titres with Day 1 plasma P-selectin. A similar relationship was found when comparing IgM and recovery P-selectin (r = 0.8; p < 0.01). The pathogen appears to contribute to platelet responses occurring during myocardial infarctions with ST-segment elevations. It is concluded that an ongoing reactivation of a chronic infection is related to increased platelet activity.