Giant cell arteritis (GCA) affecting the cranial arteries is a disease of unknown cause that causes blindness, stroke, and other morbidity. Its sudden onset and segmental distribution are suggestive of diseases that involve viral reactivation, and cranial arteries are known to be innervated by ganglia that harbor herpes simplex virus (HSV). We used a high-sensitivity polymerase chain reaction assay to test for HSV DNA in specimens from 39 consecutive temporal artery biopsies performed for suspected GCA. HSV DNA was detected in 21 (88%) of 24 histologically positive and 8 (53%) of 15 histologically negative specimens (P = .027; Fisher exact test). Analysis of 10 renal artery samples from age-matched control subjects using the same assay showed no detectable HSV DNA. We conclude that detectable HSV DNA is correlated with histologically confirmed GCA in this patient population.