Institutional review board approval of the study protocol and waiver of informed consent were obtained. This study was compliant with the Health Insurance Portability and Accountability Act. The purpose of this study was to retrospectively assess the scanning protocol and radiation dose associated with z-axis automatic tube current modulation in multi-detector row CT scanning of the chest. Fifty-three patients (mean age, 54 years; age range, 26-77 years; 25 men, 28 women) underwent 16-detector row chest CT with z-axis modulation and noise indexes of 10.0, 12.5, and 15.0 HU. Two radiologists independently compared images acquired with z-axis modulation and fixed tube current (180-300 mA) techniques for image noise, diagnostic acceptability, and depiction of peripheral bronchovascular markings. Tube current-time product was calculated for each study. There was good interobserver agreement between the two readers (kappa = 0.72). Compared with the fixed tube current technique, z-axis modulation provides acceptable image noise for chest CT, with an 18% and 26% reduction in tube current-time product at 10.0- and 12.5-HU noise indexes, respectively.