The ferret model of influenza A infection was evaluated to determine whether physical signs of influenza illness in addition to fever could be adequately followed. Ferrets were evaluated for nasal and systemic signs of influenza infection in a blinded, randomized protocol. Nasal signs were scored depending on the degree of nasal discharge and congestion. Systemic signs were evaluated on the basis of the activity level of ferrets. Nasal and systemic signs in ferrets challenged with influenza began to rise at peak virus shedding. This rise was coincident with the onset of the nasal inflammatory cell response. Nasal and systemic signs were statistically higher in challenged ferrets as compared with controls from 28 to 52 h after infection [P = 0.002 except at 28 h (P = 0.01)]. Despite precautions against influenza transmission, controls shed influenza virus associated with mild increases in nasal and systemic signs late in the course of the study. Our results suggest that severity of influenza illness can be adequately assessed in the ferret model using collective measurements of nasal and systemic signs, temperatures, and nasal cellular infiltration.