Very little is known about electrocardiogram (EKG) competency interpretation of medical residents. EKG interpretative skills are requisite skills in anyone providing general medical care. A 30-item examination containing various EKG rhythms was administered to 113 medical interns prior to assumption of any clinical responsibilities. Incorrect and correct answers were recorded and plausible explanations to common incorrect answers are discussed. This study describes a four-year qualitative study about the patterns of cardiac rhythm interpretation skills of medical interns in one university-based internal medicine residency program. Future uses and implications of the results are also discussed.