Summary Background Whether addition of fluorouracil to epirubicin, cyclophosphamide, and paclitaxel (EC-P) is favourable in adjuvant treatment of patients with node-positive breast cancer is controversial, as is the benefit of increased density of dosing. We aimed to address these questions in terms of improvements in disease-free survival. Methods In this 2 × 2 factorial, open-label, phase 3 trial, we enrolled patients aged 18–70 years with operable, node positive, early-stage breast cancer from 81 Italian centres. Eligible patients were randomly allocated in a 1:1:1:1 ratio with a centralised, interactive online system to receive either dose-dense chemotherapy (administered intravenously every 2 weeks with pegfilgrastim support) with fluorouracil plus EC-P (FEC-P) or EC-P or to receive standard-interval chemotherapy (administered intravenously every 3 weeks) with FEC-P or EC-P. The primary study endpoint was disease-free survival, assessed with the Kaplan-Meier method in the intention-to-treat population. Our primary comparisons were between dose schedule (every 2 weeks vs every 3 weeks) and dose type (FEC-P vs EC-P). This study is registered with ClinicalTrials.gov , number NCT00433420. Findings Between April 24, 2003, and July 3, 2006, we recruited 2091 patients. 88 patients were enrolled in centres that only provided standard-intensity dosing. After a median follow-up of 7·0 years (interquartile range IQR 4·5–6·3), 140 (26%) of 545 patients given EC-P every 3 weeks, 157 (29%) of 544 patients given FEC-P every 3 weeks, 111 (22%) of 502 patients given EC-P every 2 weeks, and 113 (23%) of 500 patients given FEC-P every 2 weeks had a disease-free survival event. For the dose-density comparison, disease-free survival at 5 years was 81% (95% CI 79–84) in patients treated every 2 weeks and 76% (74–79) in patients treated every 3 weeks (HR 0·77, 95% CI 0·65–0·92; p=0·004); overall survival rates at 5 years were 94% (93–96) and 89% (87–91; HR 0·65, 0·51–0·84; p=0·001) and for the chemotherapy-type comparison, disease-free survival at 5 years was 78% (75–81) in the FEC-P groups and 79% (76–82) in the EC-P groups (HR 1·06, 0·89–1·25; p=0·561); overall survival rates at 5 years were 91% (89–93) and 92% (90–94; 1·16, 0·91–1·46; p=0·234). Compared with 3 week dosing, chemotherapy every 2 weeks was associated with increased rate of grade 3–4 of anaemia (14 1·4% of 988 patients vs two 0·2% of 984 patients; p=0·002); transaminitis (19 1·9% vs four 0·4%; p=0·001), and myalgias (31 3·1% vs 16 1·6%; p=0·019), and decreased rates of grade 3–4 neutropenia (147 14·9% vs 433 44·0%; p<0·0001). Addition of fluorouracil led to increased rates of grade 3–4 neutropenia (354 34·5% of 1025 patients on FEC-P vs 250 24·2% of 1032 patients on EC-P; p<0·0001), fever (nine 0·9% vs two 0·2%), nausea (47 4·6% vs 28 2·7%), and vomiting (32 3·1% vs 15 1·4%). Interpretation In patients with node-positive early breast cancer, dose-dense adjuvant chemotherapy improved disease-free survival compared with standard interval chemotherapy. Addition of fluorouracil to a sequential EC-P regimen was not associated with an improved disease-free survival outcome. Funding Bristol-Myers Squibb, Pharmacia, and Dompè Biotec.