Introduction: In breast cancer patients treated with the anti-estrogen tamoxifen, low concentrations of the active metabolite endoxifen are associated with more disease recurrence. We hypothesized that we could increase endoxifen concentrations by induction of its formation and inhibition of its metabolism by co-administration of probenecid. Methods: We conducted a crossover study and measured endoxifen concentrations in patients on steady-state tamoxifen monotherapy and after 14 days of combination treatment with probenecid. Eleven evaluable patients were included. Results: Treatment with tamoxifen and probenecid resulted in a 26% increase of endoxifen area under the plasma concentration–time curve from 0 to 24 h (AUC0–24h) compared to tamoxifen monotherapy (95% confidence interval [CI]: 8–46%; p < 0.01), while the maximum observed endoxifen concentration increased with 24% (95% CI: 7–44%; p < 0.01). The metabolic ratio of endoxifen to tamoxifen increased with 110% (95% CI: 82–143%; p < 0.001) after the addition of probenecid. Conclusion: Probenecid resulted in a clinically relevant increase of endoxifen concentrations in breast cancer patients treated with adjuvant tamoxifen. This combination therapy could provide a solution for patients with a CYP2D6-poor metabolizer phenotype or endoxifen concentrations below the threshold despite earlier tamoxifen dose.