Background and objectives: The primary objective of sedation in the critically ill patient is to achieve security and comfort. The routine use of standardized and validated sedation scales and monitors are needed. The Richmond agitation sedation scale has been used but some patients cannot be evaluated with subjective assessment tools such as the Richmond agitation sedation scale because they lack motor responsiveness due to therapeutic paralysis or because they are receiving deep sedation. We aimed to assess the correlation of bispectral index with Richmond agitation sedation scale during dexmedetomidine sedation and evaluate the use of the bispectral index in monitoring the levels of sedation in intensive care patients. Methods: This was a single centre, prospective, clinical study. Eleven mechanically-ventilated critically ill patients, aged 17–82 (50.09 ± 17.76; mean ± SD) yr, 3 males and 8 females, APACHE II score 12.63 ± 3.90, SOFA score 3.27 ± 1.73 were enrolled in the study. Patients received a dexmedetomidine infusion of 1 μg kg−1 over 10 min followed by a maintenance infusion of 0.5 μg kg−1 h−1 for 8 h. Sedation was assessed using the Richmond agitation sedation scale and bispectral index monitoring. Heart rate, blood pressure, respiratory rate and SPO2 were monitored. Wilcoxon signed rank sum test and Spearman's rank correlation analysis were used for statistical analysis. Results: The variation of Richmond agitation sedation scale score was between 0.9 and −1.7 bispectral index varied from 65 to 75. Significant correlations between Richmond agitation sedation scale and bispectral index values were found in this study. (r = 0.900; P = 0.0001) Conclusions: Richmond agitation sedation scale levels significantly correlated with bispectral index values during dexmedetomidine sedation in critically ill patients requiring mechanical ventilation in the intensive care unit.