Purpose: To compare the image quality of digital mammograms obtained with a cassette-type retrofit digital mammography (CRM) flat panel detector (FPD) installed on an analog system and the conventional full-field digital mammography (FFDM). Materials and Methods: Digital mammograms were prospectively obtained with a CRM FPD (RoseM 2430C, DRTECH Corp) installed on an analog system (Lorad M-IV, Hologic) in 90 women (median age, 49 years) who had previous mammograms obtained with the conventional FFDM units. Ninety pairs of mammograms were evaluated by two breast radiologists. The overall image quality (including contrast and sharpness) and visibility of normal structure were evaluated using a 5-point scale (1, poor; 5, excellent). If a lesion presents, the visibility of lesions was evaluated using a 4-point scale (1, not visible; 4: high conspicuity). Results: The contrast and sharpness of CRM FPD (mean, 4.1±0.8 and 4.0±0.9, respectively) were not significantly different from those of FFDM (mean, 4.2±0.5 and 4.2±0.5; P<0.05). Of 90 women, the overall image quality was similar between the two images in 39 (43%); FFDM showed better image quality in 33 (37%); CRM FPD showed better image quality in 18 (20%) (P=0.055). There were 54 lesions (44 calcifications, 6 masses, 3 asymmetries, and one mass with calcifications) in 33 women. The difference of lesion visibility between the FFDM (mean 3.3±0.8) and CRM FPD (mean 3.4±0.8) was not statistically significant (P=0.083). Conclusion: The image quality of the mammograms obtained with CRM FPD was comparable with that of FFDM.