Background: An understanding of normal fluctuation of lymphedema over time is important to identify real change, whether it is from response to treatment or worsening of the condition. The weather is another factor that possibly influences lymphedema but has had minimal investigation to date. The purpose of this study was to determine the extent to which breast cancer-related lymphedema (BCRL) fluctuated over a 6-month period and the influence of temperature, humidity, and barometric pressure. Method and Results: Women with unilateral BCRL (n = 26) and without BCRL (n = 17) were measured on nine occasions over 6 months using a standardized protocol. Measures included self-reported arm swelling, arm volume, and extracellular fluid with bioimpedance. Daily weather data were obtained for analysis of effects on lymphedema. Neither arm volume nor extracellular fluid varied significantly for women with lymphedema; coefficients of variation were 2.3% and 3.7%, respectively. Women without lymphedema had even less fluctuation, with coefficient of variation of 1.9% for arm volume and 2.9% for ECF. Correlation of weather and lymphedema data showed that temperature was the only aspect of the weather to have any effect on BCRL, with the maximum temperature on the day before measurement slightly affecting ECF (r = 0.27, p < 0.001), arm volume (r = 0.23, p < 0.001), and self-reported swelling (r = 0.26, p < 0.001). For women without lymphedema, the weather did not affect any measure. Conclusions: Established BCRL is relatively stable over a 6-month period. Temperature was the only aspect of the weather found to impact lymphedema.