Breast cancer-related lymphedema (BCRL) is a debilitating late complication with a lack of treatment opportunities. Recent studies have suggested that mesenchymal stromal cells can alleviate lymphedema. Herein, we report the results from the first human pilot study with freshly isolated adipose-derived regenerative cells (ADRC) for treating lymphedema with 6 months follow-up. Ten BCRL patients were included. ADRC was injected directly into the axillary region, which was combined with a scar-releasing fat graft procedure. Primary endpoints were change in arm volume. Secondary endpoints were change in patient reported outcome and safety. The study is registered with ClinicalTrials.gov (NCT02592213). During follow-up, a small volume reduction was noted but was not significant. Five patients reduced their use of conservative management. Patient-reported outcomes improved significantly over time. ADRCs were well tolerated and only minor transient adverse events related to liposuction were noted. In this pilot study, a single injection of ADRC improved lymphedema based on patient-reported outcome measures, and there were no serious adverse events in the 6 months follow-up period. In addition, half of the patients reduced their use of conservative management. ADRC therapy is a promising interventional therapy for alleviating lymphedema, but results need to be confirmed in randomized clinical trials.