OBJECTIVE To investigate seasonal variability in the epidemiology and the outcome of critical illness. METHODS Retrospective analysis of prospectively collected data during the period 2001-2008. METHODS Polyvalent intensive care unit (ICU) of a tertiary hospital in the Canary Islands. METHODS Adult patients who were hospitalized in the ICU from the Emergency Department, according to the season period (spring, summer, autumn and winter). METHODS Demographic data, clinical diagnosis on ICU admission, APACHE II, need of mechanical ventilation and, finally, the mortality were collected. RESULTS During the study period, 3,115 patients, coming from the Emergency Department, were hospitalized in our ICU. Of these, 21% were admitted during the summer, an incidence rate that is statistically lower than in other seasons of the year (P<0.001). We did not find any statistically significant differences between the four groups according to the age, type of patients, severity at the moment of admission to the ICU, according to the APACHE II score or in the mortality rate in the ICU. However, significant differences were found in regards to gender. Admission of women during the summer was higher than in winter (33% vs 27%, P=0.037). Average stay in the ICU of patients admitted in the summer was similar to winter (4.9 days vs 5.8 days; P=0.052). Need for mechanical ventilation and days it was required, by diagnostic groups, were similar between the summer and the winter. The multivariate analysis did not show independent variables associated with the seasonal period in which the patients were admitted. CONCLUSIONS The stability of our climate implies that this factor does not influence the prognosis of patients who are admitted with critical illness.