Twenty-seven patients with acute frontal sinusitis, collected during a 4-month period, were examined with plain radiographics and MRI both at the time of diagnosis and one month later. Findings in the frontal and ethmoidal sinuses were recorded. Frontal sinus trephination was performed in 13 of the 27 cases. The study shows that MRI is as reliable as X-ray in diagnosing an acute paranasal sinus infection with clinical symptoms. Frontal sinusitis is not an isolated disease affecting only the frontal sinuses: Mucosal pathology can also be found widely in the ethmoidal cells, and this pathology seems to persist in many cases for weeks. MRI showed its capability in confirming these findings. MRI was not found to be an over-diagnosing method in registering the secretions and mucosal swelling during an acute paranasal sinus infection.