This retrospective study was designed to identify and assess which patient‐specific factors affect the relationship between the steady‐state trough serum quinidine concentration (SQC) measured by fluorescence polarization immunoassay and quinidine dosage. Data were obtained from 100 hospitalized patients (72 males, 28 females) receiving quinidine for atrial or ventricular arrhythmias, or both, between ages 24 and 85 years (mean age 63 yrs). Age, lean body weight, creatinine clearance (Clcr), and sex were statistically significant factors affecting this relationship; ejection fraction, total body weight, smoking history, alcohol history, recent myocardial infarction, recent surgery, elevated liver function tests, and sampling time were not statistically significant. The Clcr alone provided the most additional information in predicting SQC, and Clcr and weight provided the most additional information in predicting dosage. Currently in clinical practice, quinidine dosage adjustments are not routinely recommended for patients with renal insufficiency. These data suggest that the calculated Clcr is important in predicting both SQC and dosage when a nonspecific quinidine assay is used. This dosing model must be evaluated prospectively.