The purpose of this study was to investigate the mechanisms of aspiration with respect to the viscosity of ingested material in patients with dysphagia. Seventy patients with dysphagia underwent videofluoroscopic swallow studies (VFSS) between May 1, 2009 and September 30, 2009. Based on the findings of the VFSS, patients were divided into three groups: a thick-fluid aspiration group, a thin-fluid aspiration group, and a no-aspiration group. Kinematic analyses were performed during thick-fluid swallowing. Among our 70 patients, 23 had thick-fluid aspiration, 20 had thin-fluid aspiration, and 27 had no aspiration. A shortened duration of upper esophageal sphincter (UES) opening, a shorter interval between UES opening and peak pharyngeal constriction, and a diminished extent of laryngeal elevation were all significant risk factors for thick-fluid aspiration. A prolonged latency of the swallowing reflex, pharyngeal transit time, and the interval between bolus arrival at the vallecula and laryngeal elevation were all significant risk factors for thin-fluid aspiration. Our kinematic analysis of dysphagia employing the VFSS indicated that the mechanisms relevant to aspiration differed with respect to food viscosity.