Medically unexplained physical symptoms (MUPS) are persistent idiopathic symptoms that drive patients to seek medical care. MUPS syndromes include chronic fatigue syndrome, fibromyalgia syndrome, and multiple chemical sensitivities. When MUPS occur after an environmental exposure or injury, an adversarial social context that we call "contested causation" may ensue. Contested causation may occur publicly and involve media controversy, scientific disagreement, political debate, and legal struggles. This adversarial social context may diminish the effectiveness of the provider-patient relationship. Contested causation also may occur privately, when disagreement over the causes of MUPS takes place in the patient-provider context. These patient-provider disagreements over causation often occur because of the enigmatic nature of MUPS. We suggest that a context of contested causation may have serious negative effects on healthcare for individuals with MUPS. Context plays a larger role in MUPS care than it does for most medical care because of the uncertain nature of MUPS, the reliance of standard MUPS therapies on a potentially tenuous patient-provider partnership, and the clinical need to rely routinely on subjective MUPS assessments that often yield discordant patient and provider conclusions. Contested causation may erode patient-provider trust, test the provider's self-assurance and capacity to share power with the patient, and raise problematic issues of compensation, reparation, and blame. These issues may distract patients and providers from therapeutic goals. In occupational and military settings, the adverse impact of contested causation on the patient-provider partnership may diminish therapeutic effectiveness to a greater degree than it does in other medical settings. Contested causation therefore raises questions regarding generalizability of standard therapies for MUPS and related syndromes to these settings. Future research is needed to learn whether intuitively sensible and evidence-based MUPS therapies benefit occupational and military medical patients who are afforded care in the context of contested causation.