Aim: Prolonged stay in the intensive care unit (ICU) is associated with high mortality, morbidity and costs. Understanding the characteristics of prolonged stay ICU patients would be helpful, particularly if some factors could be modified or used in making clinical decisions. The objective of this study was to evaluate the characteristics, outcomes and cost of ICU patients with a prolonged stay (≥21 days). Materials and Methods: This retrospective study was performed in our nine-bed adult medical ICU in Gazi University Hospital. Patient data from our local database were analysed. Data were collected between April 1, 2007 and April 1, 2009. Patients with an ICU stay ≥21 days formed the study group. Results: A total of 72 patients met the inclusion criteria, with a mean age of 65.9± 16.29 years (median: 70.5 years; min: 21 years; max: 91 years), mean ICU stay of 36.96± 20.58 days (median: 30 days; min: 21days; max: 102 days) and ICU mortality of 69.5%. They accounted for 9.23% of total admissions. Intubation, reintubation, catheter insertion, catheter complications, mechanical ventilation, vasopressor support, additional investigations and procedures, changing antibiotics frequently and using expensive antibiotics had a significant association with prolonged ICU stay and costs. Conclusion: Patients with prolonged ICU stay form a small proportion of ICU patients, yet they consume a significant proportion of ICU resources. Studies identifying factors associated with prolonged length of stay can be used in targeting this group to improve resource utilisation and the efficiency of ICU care.