<i>Background:</i> The aim of the study was to investigate the clinical course of pelvic inflammatory disease (PID) and factors that could predict failed conservative treatment of PID. Additionally, the study aimed to examine the role of age and intrauterine device (IUD) use on the severity of PID. <i>Method:</i> Fifty-one women admitted to hospital with the diagnosis of acute PID were recruited. Of those, 17 patients were subsequently operated because of failed conservative treatment. All women underwent careful examination and completed a questionnaire at admission. Their clinical course was followed and the clinician responsible for the patient completed forms at admission and at discharge. Two groups were established retrospectively, those who were treated conservatively and those who underwent surgery. The outcome results were analyzed with regard to IUD use, duration of IUD use (≧5 or <5 years), and with regard to age below or above 35 years. All data were analyzed using the statistical package SAS. A p value <0.05 was considered significant. <i>Results:</i> Women who subsequently underwent surgical treatment were significantly older and significantly more frequently, current IUD users. There was no significant difference with regard to other socio-demographic characteristics. Women who subsequently underwent surgery had significantly more frequent complaints of severe abdominal pain, elevated body temperature, symptoms of peritoneal irritation, and appearance of adnexal mass. No differences were found between groups with regard to anaerobic microbiological findings, nor with regard to the finding of <i>Actinomyces</i>. IUD use and age ≧35 were found to be highly significant risk factors for surgery in patients with PID. <i>Conclusion:</i> Age over 35 years and IUD use, independently of each other, were factors strongly associated with an increased risk of surgery for PID as a result of failed conservative treatment.