Objectives: To study platelet functions in various clinical conditions commonly seen in intensive care units as sepsis syndrome, low output state, and respiratory failure, study the effect of disease severity of these conditions (using APACHE II scoring system) on platelet functions, and to study platelet dysfunction as a determinant factor for survival or mortality in the ICU stay. Patients: thirty-two patients were included in the study and ten normal individual as control. Patient group was subdivided into sepsis (n = 13), low output (n = 9), and respiratory failure (n = 10) subgroups. Patient group was also subdivided into group with mild disease illness (15 patients), and group with severe disease illness (17 patients), according to APACHE II score below 12 and above 12 respectively. Patients were also divided according to their outcome into survivor group (14 patients) and non-survivor group (18 patients). Methods: Plasma rich platelets were tested for percentage of platelet aggregates induced with collagen, ADP, arachidonic acid, and ristocetin by photo-optical platelet aggregometry, and glycoprotein expression by flow cytometry. Plasma poor platelets were tested for beta thromboglobulin levels. Results: Plasma beta thromboglobulin levels were taken as a good and sensitive marker for platelet activation. Beta thromboglobulin levels were 83.3 + 21.3 IU/ml, 82.6 + 27.4 IU/ml, 62.7 + 39.3 IU/ml, and 24.3 + 1.82878 for sepsis, low output, respiratory failure, and control groups respectively with significant p value (< 0.001, < 0.001, and 0.006) for sepsis, low output, and respiratory failure groups respectively in comparison to control group. Beta thromboglobulin levels were 80.4 + 33.4 IU/ml and 73.4 + 27.6 IU/ml for patients with mild diseases and severe diseases respectively with insignificant p value in comparison to each other. Beta thromboglobulin levels were 80 + 35.3 IU/ml and 74.1 + 26.3 IU/ml for survivors and non-survivors respectively with insignificant p value in comparison to each other. APACHE score was 9.29 + 2.55 and 17.44 + 6.22 with significant p value (< 0.001) for survivors and non- survivors respectively. Conclusion: Platelet dysfunction in the form of platelet activation may occur in critically ill patients with different diagnoses as sepsis, low output, and respiratory failure. Platelet activation found was in the form of increased plasma beta thromboglobulin levels. Contrary to expectation, platelet function tests as, platelet aggregation detected by photo-optical aggregometry technique and glycoprotein expression by flow cytometry technique used in critically ill patients with sepsis, low output, and respiratory failure showed insignificant results for these patients as compared to normal control individuals.Platelet dysfunction does not correlate with the severity of the diseases. Although platelet dysfunction constitutes a part of the pathophysiology of these diseases, patients’ survival or mortality do not rely on platelet dysfunction, but rely directly to the severity of the diseases which significantly correlates with the APACHE II score.