Breast cancer‐related lymphedema (BCRL) is a chronic swelling of the arm that sometimes follows breast cancer treatment. Clinically, both skin and subcutis are swollen. Edema is considered to be predominantly subcutaneous and of an even distribution. The purpose of this study was to quantify the degree and uniformity of skin and subcutis swelling around the forearms of women with BCRL. Ten women with BCRL were recruited. Both forearms were examined using 20 MHz ultrasound to visualize the skin and 7 MHz ultrasound to visualize the subcutis. Skin thickness was between the bottom of the entry‐echo and the skin–subcutis boundary. Subcutis thickness was measured between the skin–subcutis boundary and the subcutis–muscle boundary. Both average skin thickness (1.97 ± 1.00 mm) and average subcutis thickness (10.32 ± 5.63 mm) were greater in the ipsilateral arm than in the contralateral arm (skin 1.12 ± 0.14 mm, subcutis 5.58 ± 2.04 mm, p < 0.01, t‐test). The degree of increase in skin thickness did not vary around the arm (p > 0.05, ANOVA), while the degree of increase in subcutis thickness did vary (p < 0.05). Skin thickness correlated negatively with subcutis thickness in the contralateral arm, but correlated positively in the ipsilateral arm. The skin and subcutis are thickened in the ipsilateral arm of patients with BCRL. Skin thickness is increased uniformly around the arm and correlates strongly with the degree of swelling, while subcutis swelling varies. The measurement of skin thickness using ultrasound may form a useful clinical tool in the diagnosis of lymphedema and also aid further investigation of therapeutic techniques.