Background. Dissection of posterior triangle apex (APEX) is a surgical step in supraomohyoid and lateral neck dissections. The prevalence of lymphatic metastases at this site and the clinicohistopathologic conditions that influence their occurrence have not been established. We have evaluated the prevalence and the risk factors for cervical metastases in lymph nodes of the APEX. Methods. Sixty-two neck dissections were performed in 51 patients with squamous cell carcinoma of the oropharynx, hypopharynx, oral cavity, glottic larynx, and supraglottic larynx or with primary occult tumor. We correlated the presence of positive metastases in the APEX with the neck level involved either clinically (CLIN) or histopathologically (H/P) and with the number of CLIN- or H/P-positive neck levels with metastases. The prevalence of metastases in the APEX in elective (NO) and therapeutic (N+) neck dissections was also compared. This prevalence was also compared with that for each neck level. The histopathologic comparisons between the APEX and the neck levels were calculated for NO, N+, and all neck dissections. The primary site of tumor was correlated with the presence of H/P-positive nodes in the APEX. Results. The overall prevalence of lymphatic metastases in the APEX was 6.5%. The prevalence in NO neck dissections was 2.3% and in N+ neck dissections it was 16.7%. The prevalence of lymphatic metastases in the APEX for primary tumors of pharynx was 23.1%, for the oral cavity it was 3.6%, and it was 0% for other sites. Metastases in the APEX were not influenced by the neck level with CLIN or H/P metastases in N+ necks. The number of CLIN- or H/P-positive neck levels had no influence on histopathologic metastases in the APEX. Factors that influenced metastases in the APEX were positive histopathologic metastases at level II for NO neck dissections and positive histopathologic metastases at level II or III for all neck dissections. All the comparisons were analyzed using Fisher's or Poisson's test. Conclusions. The prevalence of histopathologic metastases in the APEX in N+ necks is 7.3 times greater than that of NO necks and for primary tumors of pharynx it was 6.4 times greater than for the oral cavity and significantly greater than for the larynx. Histopathologic metastases at level II for clinically NO necks and histopathologic metastases to level II or III for all neck dissections are risk factors for metastases in the APEX. The number of positive levels did not influence the prevalence of metastases in the APEX. There are no isolated metastases in the APEX of the posterior triangle. (C) 2000 John Wiley & Sons, Inc.226564571