Blood rickettsial titers, skin biopsy results, and circulating endotoxin measurements were correlated with the clinical course of disease in patients with Rocky Mountain spotted fever (RMSF). Nine of 11 patients with documented RMSF had Rickettsia rickettsii isolated from plasma samples. Of the eight patients in whom rickettsial titers were measured, seven had 10(0.7) to 10(1.2) median tissue culture infective doses (TCID50) per milliliter; all seven had mild to moderately severe disease. One patient with fulminant, fatal untreated RMSF had 10(3) TCID50/mL of postmortem plasma. Two patients from whom rickettsiae were not isolated had positive direct immunofluorescent stains of skin biopsy material for R rickettsii. Circulating endotoxin was present in two patients, one with documented rickettsemia and one with a positive skin biopsy alone. Only low levels of circulating rickettsiae are present in patients with moderately severe disease. Measurement of plasma endotoxin is not useful in the early diagnosis of RMSF.