With the development of new antimicrobial agents, the incidence of peritonsillar abscess (PTA) is on the decline. PTA is still often encountered in general practice, however, where it requires immediate diagnosis and treatment. Because the internal carotid artery runs medially to the medial parapharyngeal space, damage to nearby vascular or other structures is a surgical risk of PTA. We used contrast computed tomography (CT) from PTA patients to investigate the anatomical relationship between the abscess and parapharyngeal space, and to determine safe surgical sites. We observed 31 patients with PTA--19 men and 12 women--between February 1997 and April 1999, all examined by contrast CT and undergoing drainage or incision. The average age was 30.7 years (range: 12-54 years). The abscess was on the right side in 20 cases and on the left side in 11. We determined the sites of the abscess and carotid artery, internal jugular vein, and surrounding soft tissue density area including nerves in the parapharyngeal space based on the angle and distance from recognizable anatomical structures in CT scans. The anterior margin of the parapharyngeal space was 29 +/- 5 mm posterior from the upper posterior alveolar margin. The medial margin of that space was at 15 +/- 2 degrees laterally from the midline of the incisors, and 24 +/- 4 mm laterally from the midline sagittal plane. The internal carotid artery was located medially to the parapharyngeal space, running on the sagittal plane containing the upper posterior alveolar margin. The distance from the anterior margin of the parapharyngeal space to the posterior wall of the PTA was 9 +/- 4 mm, and the distance to the anterior wall of the abscess (including the pharyngeal mucosa) was 31 +/- 5 cm. The relationship between the upper posterior alveolar margin and midline sagittal plane was useful for determining the site of the parapharyngeal space. Because the internal carotid artery is located on the same sagittal plane as the upper posterior alveolar margin, when conducting drainage or incision of PTA, we should advance sagittaLly from the point of incision to a depth of no more than 20 mm. If the tip of the instrument is kept medial to the sagittal plane of the upper posterior alveolar margin, effective treatment should be achievable without the risk of vascular damage.