Background: Lymphedema is a common complication after mastectomy in women with breast cancer. Several methods have been described to assess and diagnose lymphedema, one of the most studied being the perimeter and ultrasonography. However, the reliability of these methods and the correlation between them are still controversial. The aim of this study was to analyze the reliability of cytometry and ultrasound imaging in the assessment of lymphedema after mastectomy in women with breast cancer and to study the correlation between them. Methods and Results: A cross-sectional study was conducted in 29 women with mastectomy after breast cancer. Lymphedema in the arm was measured both with cytometry and ultrasonography. Reliability was calculated with intraclass correlation coefficient. The correlation between the two methods was carried out with the Pearson correlation coefficient. Both cytometry (M1: α = 0.999, ICC = 0.996; M2: = α = 0.998, ICC = 0.994) and ultrasonography (M1: α = 0.992, ICC = 0.976; M2: = α = 0.991, ICC = 0.973) are reliable methods to assess lymphedema in the arm. No significant correlation was found between them ( p > 0.05). Conclusions: Cytometry and ultrasonography appear to be adequate for the measurement of edema in women with breast cancer after mastectomy. However, for an accurate measurement of lymphedema, these measurements should not be used interchangeably.