Endotoxemia occurs when intestinal ischemia allows bacterial lipopolysaccharide to translocate from colonic flora into the bloodstream, which triggers release of cytokines that can cause hypotension, rigors, fever, shock, and even death. Recently, blood endotoxin levels were shown to be higher in athletes needing medical attention (330 pg.ml-1) than in their competitors with similar performances (81 pg.ml-1). Though there were no data showing that these athletes had elevated core temperatures or severe illness, speculation followed that endotoxin may play a causal role in heat stroke. We examined the relationship between endotoxemia and mild post-exertional illness in 39 cyclists after a 100-mile ride. Thirteen cyclists had at least one of the following: orthostatic hypotension, rigors, nausea, vomiting, diarrhea, or syncope. Only 2/26 case-controls had any of these symptoms. Data were collected on vital signs, hemoglobin, sodium, creatine kinase, creatinine, and uric acid. Endotoxin titer was determined by chromogenic assay; tumor necrosis factor alpha (TNF-alpha) titer was determined by ELISA. One ill cyclist had an endotoxin level of 330 pg.ml-1, one control had an endotoxin level of 150 pg.ml-1, but endotoxin level was < or = 64 pg.ml-1 in all others. Comparison of pre- and post-ride data showed that controls increased creatine kinase activity (154 +/- 34 vs 561 +/- 191 IU.dl, P < 0.05), creatinine concentration (1.5 +/- 0.0 vs 1.6 +/- 0.0 mg.dl-1, P < 0.05), and uric acid concentration (5.4 +/- 0.3 vs 6.3 +/- 0.3 mg.dl-1, P < 0.05). Ill cyclists had lower serum sodium than post-ride controls (138 +/- 2 vs 142 +/- 0.6 mEq.l-1, P < 0.05), but there were no differences between groups in CK, creatinine, or uric acid. These findings suggest that endotoxemia may complicate, but does not cause mild post-exertional illness in cyclists.