This article takes the phenomenon of phantom limb pain (PLP), and a therapeutic technology designed to treat it, as springboards to critically consider a transformation: from deeply subjective experiences into quantitative data. Drawing upon ethnographic fieldwork on neuroprosthetic development, I examine an international clinical trial coordinated in Sweden using neuromuscular activation, machine learning, and virtual reality to treat PLP. I excavate the trial’s underlying fundaments and tools, tracing how they define, produce and record changes in an individual’s pain along the course of treatment, a process I call the ‘datafication of pain.’ Moving beyond the representational problematic of pain as simultaneously subjective experience and object of medical intervention, I ask: What gets left out, in this process of datafication? And what gets created in the void it leaves? I argue that the experimental paradigm of datafication elides certain key dimensions of pain itself, particularly its relational dimensions, and surfaces new pain-experiences in-situ. The stakes of this elision and surfacing not only impact the data produced, but also the ethics of actual lived, embodied experiences of pain itself. In leaking out of the experimental apparatus, the excess of pain becomes an artifact of the experimental process, as opposed to merely its object. This article examines the relational dimensions—of both the experimental process and phantom limb pain at large—elided by the data-gathering apparatus itself.