Explaining symptoms by the most likely cause is a process during which hypotheses are activated and updated in memory. By letting participants learn about causes and symptoms in a spatial array, we could apply eye tracking during diagnostic reasoning to trace the activation level of hypotheses across a sequence of symptoms. Fixation proportions on former locations of possible causes reflected the causal strength of initial symptoms, a bias towards focal hypotheses, and the final diagnosis. Looking-at-nothing revealing memory activation consistent with process models of diagnostic reasoning was stable even after one week.