A diagnosis-antibiotic prescribing study was performed in 5 counties in Sweden during 1 week in November in 2000 and 2002 respectively. As a part of the study, the use and results of C-reactive protein (CRP) tests in relation to duration of symptoms and antibiotic prescribing in 6778 patients assigned a diagnosis of respiratory tract infections were analysed. In almost half (42%) of the patients, a CRP test was performed. The majority of CRP tests (69%) were performed in patients assigned diagnosis upper respiratory tract infection, where the test is not recommended. Overall, there was a minor decrease in antibiotic prescribing when CRP was used (41%), in comparison to 44% of the patients where no CRP was performed (p < 0.01). Patients assigned diagnoses implying a bacterial aetiology were prescribed antibiotics irrespective of result of CRP or length of symptoms before consultation. For patients assigned viral diagnoses, antibiotic prescribing increased with increasing duration of symptoms and increasing value of CRP. The use of CRP decreased antibiotic prescribing in patients assigned to viral diagnoses and with longstanding symptoms (p < 0.001). However, 59% of the patients assigned viral diagnoses with CRP > or = 25 received antibiotics, which seems to indicate a misinterpretation of CRP and a non-optimal use of antibiotics.