Objective: To systematically review clinical evidence gathered by direct observation of medication errors in adult patients in intensive care units. Methods: Articles published between 1985 and 2008 in English-language journals indexed by the Cumulative Index for Nursing and Allied Health Literature and PUBMED were searched for studies on medication errors made by intensive care unit nurses. Studies in which errors were detected via direct observation were included. Results: Six studies met the inclusion criteria, and error incidence varied considerably among them. Wrong dose, wrong administration time and rate, and dose omission were the most common errors. Antibiotics, electrolytes, and cardiovascular drugs were commonly associated with errors, but the evidence about factors contributing to errors was inconclusive. Increased monitoring was the most common consequence of medication errors, whereas life-threatening and fatal adverse events were rare. Conclusions: Identification of patterns and characteristics of medication errors can guide preventive interventions. Factors contributing to errors, as well as drugs and error types associated with severe adverse events, deserve further investigation.