Invasive squamous cell carcinoma of the vulva with ≤1 mm stromal invasion is classified as stage 1A. Cancer staging systems state that the depth of invasion should be measured from the epithelial-stromal junction of the adjacent most superficial dermal papilla to the deepest point of the invasive tumor. Measurement of the depth of invasion guides patient management. Even though this measurement is critical, no studies have reported the reliability among pathologists for determining the cutoff point of ≤1 mm stromal invasion in vulvar cancer. We assessed agreement among pathologists for determining whether a vulvar tumor is invasive, for the depth of invasion, and for tumor thickness. Forty-five cases of vulvar squamous cell carcinoma with a depth of invasion of ≤5 mm were chosen. Eleven gynecologic pathologists independently reviewed the slides and, for a subset of cases, pictorially recorded measurements on photographs. The number of cases that were reported as invasive by the 11 pathologists ranged from 21 to 44. The number of cases that were reported as showing a depth of invasion of ≤1 mm ranged from 7 to 27. Eight pathologists provided measurements for all lesions reported as invasive, the remaining 3 pathologists stated that they were unable to measure 2, 7, and 16 lesions, respectively. Mean κ for diagnosing vulvar carcinoma as invasive was 0.24 and for measuring the depth of invasion and thickness was 0.51 and 0.49, respectively. There was only fair agreement in determining whether the lesion was invasive. In cases in which pathologists agreed upon the diagnosis of invasion, agreement on depth was moderate. When using the recommended cancer staging method, interpretation of the location of the most superficial dermal papilla varied among pathologists. Measuring thickness did not improve agreement. This is the first study that has assessed the reliability of the diagnosis of invasion in vulvar cancer among gynecologic pathologists, the interobserver agreement for reporting the critical 1 mm threshold of depth of stromal invasion, and the way in which the International Federation of Gynecology and Obstetrics method is used by pathologists.