Since the Gulf War, investigation continues of symptoms and illnesses among its veterans. Yet, identifying a specific "Gulf War Syndrome" remains elusive. With new disease entities, causal associations are relatively easily established when the condition is serious, verifiable, and has excess disease rates in specific groups. In common conditions, many excess cases are required to establish association with a specific exposure. Establishing causality in syndromes with variable symptoms is difficult because specific diagnostic algorithms must be established before causal factors can be properly investigated. Searching for an environmental cause is futile in the absence of an operational disease case definition. Common subjective symptoms (without objective physical or laboratory findings) account for over one-half of all medical outpatient visits, yet these symptoms lack an identified physical cause at least one-third of the time. Our medical care system has difficulty dealing with disorders where there is no identified anatomic abnormality or documented metabolic/physiological dysfunction.