To determine whether adherent material found on the walls of the paranasal sinuses during common colds might be fibrin clot, we examined the nasal fluid (a surrogate for sinus secretion) of 11 young adults with experimentally induced rhinovirus colds and that of 4 control subjects for the presence of fibrin. The mean concentration (+/- the standard error) of insoluble fibrin (measured as D-dimer) in subjects with rhinovirus colds increased from a baseline level of 0.8+/-0.4 microgram/mL to a peak of 2.4+/-0.7 microgram/mL (P=.0008) on day 4 after inoculation of the virus, but the fibrin concentration remained at baseline levels in the 4 uninfected control subjects.