BACKGROUND: Direct monitoring of arterial blood pressure provides continuous, real-time information about patients' physiological status. Critical care nurses set up and maintain monitoring systems and use the obtained data to guide clinical decisions. Inaccurate measurements may lead to misdiagnosis and mismanagement. PURPOSE: To describe critical care nurses' knowledge in 3 content areas related to direct monitoring of arterial blood pressure: physiology, technical aspects, and waveform and data interpretation. METHODS: Via poster advertisements, 391 critical care nurses in 6 intensive care units at 2 hospitals were invited to complete an 18-item, criterion-referenced questionnaire on monitoring arterial blood pressure and a demographic data sheet. Summary statistics were used to analyze data from 68 subjects. Analysis of variance was used to determine if total and subset scores differed among demographic subgroups. RESULTS: Scores ranged from 11.1% to 61.1% correct answers, with a mean of 36.7% (SD, 11.8%). Item analysis indicated a knowledge deficit in all content areas at all cognitive levels. Questions with highest scores addressed waveform damping and using mean arterial pressure to guide treatment; lowest scores were related to dynamic response characteristics and reflected pressure waves. Mean scores did not differ among demographic subgroups. CONCLUSIONS: The results suggest a general knowledge deficit in arterial blood pressure monitoring. This study should be replicated on a larger scale to validate its findings and to improve the validity and reliability of the research tool. National research-based standards of practice for hemodynamic monitoring should be developed and disseminated among critical care nurses.