The idea behind this thesis is to present how ARS and especially acute maxillary sinusitis in adults is diagnosed and treated in general practice. The study extends over many years, beginning with the first survey in 1991. Based on doctors' answers, we then investigated the diagnostic values ​​of the symptoms, signs and examinations which the doctors reported using. All patients over 18 years suspected of acute maxillary sinusitis were included consecutively and only once and, after a clinical examination with the GP, they were offered the opportunity to enter into the prospective study referred to acute CT scan and by changes in the CT, immediately referred to sinus puncture. Both examinations were conducted at Aalborg Hospital. The disease was found most frequently in younger and 2/3 were women. The reason for this gender difference is unknown. We have assessed the diagnostic values of the symptoms, objective findings and investigations ​​using 3 different reference standards: sinus puncture, microbiological diagnosis and CT scan described in three articles. In all examinations, it appeared that the usual signs and symptoms of acute maxillary sinusitis occur almost equally often and with a few exceptions in patients, with and without pus in the sinus cavities. Pain in the sinus cavities occurring in 95% of patients, and only elevated levels of CRP and ESR are significantly and independently associated with pus in the sinus cavities. This finding is surprising, because they are two nonspecific markers. CRP tested by near-patient testing has, within the investigations period, been introduced in general practice, and from 1999 the doctors also get reimbursed for performing the test. We have on this background originally defined a clinical criterion with pain over the sinuses accompanied by elevated values ​​of CRP and/or ESR giving a sensitivity of 0.82, specificity 0.57, ppv 0.68 and npv 0.74. But looking at the ROC curve we suggest that a more clinical relevant diagnose will be based on use of CRP alone, as the test can be made easily and fast while the patient is in the clinic compared to the use of ESR. The disease is over-diagnosed in general practice. In only 53% of patients, who the GP suspected of having acute sinusitis, was there detected pus or mucopus at the sinus puncture, furthermore the patients' statements that they had had sinusitis was significantly negatively associated with current acute maxillary sinusitis. Almost all patients are prescribed topical treatment to the nose in the form of vasoconstrictor, and 50-70% also antibiotics. The most common bacteria that can be isolated are S. pneumoniae and H. influenzae. For many years the first drug of choice has been penicillin V, and treatment with penicillin V has followed Scandinavian recommendations. However, the resistance patterns in respect of H. influenzae have changed over the years and if the dominant flora is H. influenzae, then oral penicillin is not sufficient anymore, and should be replaced by amoxicillin with or without clavulanate. It is reported that the MIC of penicillin V is too high, such that oral dosage cannot provide sufficiently high concentrations. However, in daily clinical practice the doctor does not have the possibility to decide whether the infection is caused by either S. pneumoniae or H. influenzae, unless a sinus puncture is performed and it is not considered as a standard procedure. The recommended treatment is therefore starting with penicillin V, and at treatment failure switching to amoxicillin with or without clavulanate. It is well known that URTI's can exacerbate a chronic pulmonary disease - like asthma - in allergic patients, but this influence is also demonstrated as described in article 6 where ARS in adults without any sign of chronic lung disease or allergy is accompanied by a temporary reduction lung function. Future research should focus on the use of CRP in general practice, analysing cost-effectiveness of the use of CRP patient outcome in relation to antibiotic treatment, clarification of ARS as a female disease, and a detailed exploration of the relationship between URTI's and impaired lung function in lung-healthy patients.