Problem-based learning (PBL) has become a distinct approach in learning. To assess the effectiveness of PBL in teaching ethics to medical students and the effect it has on their ethical reasoning, we decided to hold a PBL ethics workshop and then evaluate them based on pretest and posttest. This quasi-experimental comparative study in Shiraz University of Medical Sciences was designed based on pretest-posttest nonequivalent groups. After obtaining their consent, 120 health care providers were randomly selected and assigned in 4 groups and then asked to attend the ethics workshop. For measuring the participants’ ethical reasoning through pretesting and posttesting, a case study consisting of 21 multiple-choice questions (cognitive domain–apply level) was performed. Data were then collected using a questionnaire, which was completed by the participants and analyzed using SPSS software (version 17). The comparison between the participants’ attitude and knowledge toward ethics before and after the workshop revealed that all indices in the cognitive domain were changed (P < .001). The scores of pretest and posttest were significantly different. As to the results of our study, the PBL groups showed a more positive learning attitude and higher motivation in comparison with the control group who were subjected to traditional-based method of learning. The result of our study suggests that PBL can and should be used as an alternative method in teaching ethics in medical students because it is more effective and motivates the students.