The purpose of this study was to quantify the degree to which fibromyalgia patients perceive the cause of their pain to be inexplicable or difficult to understand. The author developed two simple Likert scales, Understand Pain Scale and Explain Pain Scale, which ask the subject to indicate the degree to which they are able to, respectively, understand the cause of their pain and to explain the cause of their pain to others. A total of 104 subjects who met the 1990 American College of Rheumatology Diagnostic Criteria for fibromyalgia (FM group), and 272 subjects with widespread pain who did not meet these criteria (non-FM group) completed these two instruments. On the Understand Pain Scale, 67.3 % of FM subjects endorsed either the item “understand very little about the cause of my pain (the reason I have pain)” or “cannot understand at all the cause of my pain (the reason I have pain)”. By comparison, 16.2 % of the non-FM group with widespread pain endorsed either of these Understand Pain Scale items. On the Explain Scale, 84.6 % of fibromyalgia subjects endorsed either the item “can very little or not very often explain the cause of my pain (the reason I have pain) to others” or “cannot at all explain the cause of my pain (the reason I have pain) to others”. In contrast, 21.7 % of non-FM group subjects with widespread pain endorsed either of the aforementioned items. Compared to other patients with chronic, widespread pain, fibromyalgia patients report a much greater degree of difficulty in understanding the cause of their pain and explaining the cause of their pain to others. This phenomenon may reflect the narrative of “inexplicability” in fibromyalgia patients that distinguishes them from other widespread pain populations.