Pavlidis N, et al. - Researchers sought to characterize cancer of unknown primary (CUP) in adolescents and young adults (AYA), aged 15-39 years by assessing patient information from the Surveillance, Epidemiology, and End Results (SEER) registry, 1990-2015, including age, gender race, pathological classification, socioeconomic status, incidence rates, modeled relative risks and survival of AYA patients with CUP; 907 met the definition of standard pathology classifications. Neuroendocrine, squamous cell, and poorly differentiated carcinomas were seen in most AYA patients with CUP. The greatest risks of CUP were seen in AYA living in areas with the highest socioeconomic status level; AYA living in non-metropolitan areas had a reduced risk of CUP. Since the early 1990s, the incidence of differentiated neoplasms has been decreasing more slowly vs undifferentiated neoplasms. Eleven months was the median overall survival (OS); squamous CUP had the longest median OS at 16 years.