Background. The drug levels and clearances of imipenem in critically ill patients are not comprehensively described in current literature, yet it is vital that adequate levels be achieved for therapeutic success. Objectives. To determine the proportion of critically ill patients treated with imipenem/cilastatin with sub-therapeutic imipenem plasma levels, and to compare the clinical outcomes of those patients with therapeutic levels with those who had sub-therapeutic levels. Methods. Trough imipenem plasma levels of 68 critically ill patients from a surgical intensive care unit were measured using a validated high-performance liquid chromatography method. Imipenem trough levels were compared with the minimum inhibitory concentration (MIC) of the causative bacterial agents, based on a target value of 100% time above MIC (ƒT >MIC). Results. The proportion of participants with sub-therapeutic imipenem levels was 22% (95% confidence interval (CI) 13% - 34%). The 14- and 28-day mortality rates in the sub-therapeutic group were 33% and 40%, respectively, compared with 19% (p=0.293) and 26% (p=0.346), respectively, in the therapeutic group. Sub-therapeutic imipenem plasma levels are associated with adjusted hazard ratio of 1.47 (95% CI 0.55 - 3.91). Conclusions. The lower proportion of critically ill patients with sub-therapeutic imipenem plasma levels in this study compared with previous studies may be attributed to the practice of higher dosages and the administration method of extended infusions of imipenem/ cilastatin in our setting. The results demonstrate a trend of higher mortality in patients with sub-therapeutic imipenem levels, although the results were not statistically significant at this sample size.