AbstractAdjuvant anthracyclines and taxanes reduce recurrence and death in early‐stage breast cancer (EBC) patients, but toxicity is a concern. Studies show conflicting results on the correlation between body mass index (BMI) and outcomes. Limited data exist on the efficacy of adjuvant taxanes among BMI categories and the impact of different taxane‐based chemotherapies (paclitaxel vs. docetaxel) on disease recurrence. Here, we present a pooled analysis of 13,486 EBC patients treated with adjuvant anthracyclines ± taxanes from seven GEICAM and TRIO trials (1996–2008) conducted. Patients were classified into four BMI categories: normal (<25.0), overweight (25.0–29.9), obese (30.0–34.9), and severely obese (≥35.0). BMI was evaluated as a predictive factor for the efficacy and toxicity of taxane‐based chemotherapy. Our results show the following findings: patients' distribution by BMI was 44% normal, 33% overweight, 16% obese, and 8% severely obese. Seventy‐nine percent received taxane‐based chemotherapy. Ten‐year invasive disease‐free survival (iDFS) was 71%, 70%, 68%, and 64% for normal, overweight, obese, and severely obese patients, respectively. Obese and severely obese patients had significantly worse outcomes (HR 1.15 and 1.29, respectively). Invasive disease‐free survival with docetaxel vs. non‐docetaxel was significant in the normal BMI group, while iDFS with paclitaxel was significant in the obese group. Relevant toxicity was observed in 5%, 5.5%, 5.9%, and 9.3% of normal, overweight, obese, and severely obese patients who received docetaxel. In conclusion, heavier EBC patients had a worse prognosis with adjuvant taxane‐based chemotherapy. Normal BMI patients benefited more from docetaxel, while obese patients benefited more from paclitaxel.