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The objective of our study was to evaluate the impact of body mass index (BMI) on dose, diagnostic performance, and image quality of a low-dose CT examination for renal colic.This retrospective study included all patients who underwent a low-dose CT examination for renal colic performed during the year 2012 with automatic tube current modulation, adaptive statistical iterative reconstruction, and a low tube voltage (kV). Three readers independently reviewed all images for the presence of renal colic and evaluated diagnostic confidence and image quality. The results and doses were compared among patients grouped by body mass index (BMI) and between patients with a BMI25 and those with a BMI≥25.Eighty-six patients were included in the study: 39 patients had a BMI25 and 47 had a BMI≥25. No statistically significant difference was found between the accuracy rates for the diagnosis of renal colic when the rates of the three independent readers were averaged for both BMI groups (95.7% vs 96.4%, respectively; p=0.83). Image quality and diagnostic confidence scores were significantly better for patients with a BMI≥25 than for patients with a BMI25 (mean image quality score, 3.7 vs 3.4, p0.001; mean diagnostic confidence score, 2.8 vs 2.5, p0.001). The mean radiation dose for patients with a BMI25 was 2.4 mGy compared with 3.7 mGy for patients with a BMI≥25 (p0.001).The diagnostic performance of our low-dose CT protocol for renal colic was excellent for all patients, and image quality and diagnostic confidence were significantly better for patients with a BMI≥25. However, our protocol also required exposure to a greater dose of radiation for these overweight and obese patients.