Sinusitis is a common problem in general practice, but diagnosis is often difficult. The greatest clinical challenge is to distinguish between purulent and nonpurulent sinusitis. This differential diagnosis has consequences for treatment. A questionnaire was sent to a randomized sample of Norwegian general practitioners and to all ear, nose and throat specialists in private practice. 79% of the general practitioners felt that diagnosis is more uncertain for sinusitis than for otitis media. When anamnestic questions and clinical findings were grouped, it was shown that general practitioners consider pain to be much more relevant in the differential diagnosis than purulent secretion. The ear, nose and throat specialists consider pain and purulent secretion to be equally important in the differential diagnosis. Pain in cheek and forehead, and pain in the upper teeth are regarded as the most relevant single anamnestic symptoms. Duration of illness is considered to have little differential diagnostic relevance. Among supplementary tests only C-reactive protein is regarded as helpful. X-ray and erythrocyte sedimentation rate are evaluated as far less relevant.