STUDY OBJECTIVES: To compare the case mix and outcomes of male and female patients admitted to intensive care units for a wide range of conditions. DESIGN: Cross sectional study of prospectively collected data. SETTING: The Intensive Care National Audit and Research Centre, (ICNARC) Case Mix Programme. PARTICIPANTS: 46 587 admissions to 91 units across England, Wales, and Northern Ireland. MAIN RESULTS: No gender differences were found in case mix on admission or in mortality for five conditions (cardiac arrhythmia, chronic obstructive airways disease, asthma, self poisoning, and seizures). There was some evidence of horizontal and vertical inequity for female patients with myocardial infarction and with neurological bleeding. Vertical equity was not achieved for male pneumonia and ventricular failure patients and for women with primary brain injury. CONCLUSIONS: This study demonstrated, for the first time, possible inequitable use of intensive care for patients with certain conditions. This may be secondary to gender bias and can result in either over-treatment in the favoured group, or under-treatment in the neglected group. It would therefore be pertinent to re-examine these findings using other databases, and to further investigate the causative factors, including gender bias.