Hops (Humulus lupulus L.) are used in the brewing of beer, and hop extracts containing prenylated compounds, such as xanthohumol (XN) and 8-prenylnaringenin (8-PN), are under investigation as dietary supplements for cancer chemoprevention and the management of hot flashes in menopausal women. To facilitate clinical studies of hop safety and efficacy, a selective, sensitive, and fast ultra-high-pressure LC (UHPLC) tandem MS method was developed and validated for the simultaneous determination of the hop prenylflavonoids XN, isoxanthohumol (IX), 6-prenylnaringenin (6-PN), and 8-PN in human serum. The analytical method requires 300 microL of human serum, which is processed using liquid-liquid extraction. UHPLC separation was carried out in 2.5 min with gradient elution using an RP C18 column containing 1.6 pm particle size packing material. Prenylflavonoids were measured using negative ion electrospray MS with collision-induced dissociation and selected reaction monitoring. The method was validated and showed good accuracy and precision with an LOQ of 0.50 ng/mL for XN (1.4 nM) and 1.0 ng/mL for 6-PN, 8-PN (2.94 nM), and IX (2.82 nM) in serum.