Japanese spotted fever (JSF) is severe and can progress to disseminated intravascular coagulation (DIC) with a poor prognosis. We considered whether patient factors are related to serious complications. Between August 1999 and March 2009, all patients with JSF and retrievable clinical data (age, gender, length of hospital stay, medication, comorbidities), vital signs (blood pressure, heart rate, temperature), and laboratory test results [blood cell count, liver function, renal function, electrolytes, blood sugar, C-reactive protein (CRP), CRP normalization period, and aspartate aminotransferase normalization period] from the Integrated Intelligent Management System (IIMS) database, were retrospectively analyzed by logistic regression. There were 51 JSF patients (24 men, 27 women) with a mean age of 63.0 years. Six patients (11.8%) had DIC, but there were no in-hospital deaths. The time between fever onset and initiation of medication was approximately 5 days, but this delay was not associated with disease severity. We identified values correlating with disease severity (p < 0.1) by univariate analysis and then applied logistic regression. We found renal dysfunction [serum creatinine (Cr) ≥1.5 mg/dl] at the time of initial presentation to be predictive of DIC. Cr was also predictive of a prolonged disease course. In patients with JSF, renal function must be carefully monitored when determining clinical management.