Abstract Background This randomised Phase II study assessed the activity and safety of concurrent chemoradiotherapy (CRT) and lapatinib followed by maintenance treatment in locally advanced, unresected stage III/IVA/IVB head and neck cancer. Patients and methods Patients were randomised 1:1 to concurrent CRT and placebo followed by placebo or concurrent CRT and lapatinib followed by lapatinib. Treatment continued until disease progression or study withdrawal. Primary end-point was complete response rate (CRR) by independent review 6 months post-CRT. Results Sixty-seven patients (median age 56 years; 97% Eastern Cooperative Oncology Group performance status ⩽1; 82% stage IV) were recruited. CRT dose intensities were unaffected by lapatinib: median radiation dose 70Gy (lapatinib, placebo), duration 49 (lapatinib) and 50 days (placebo); median cisplatin dose 260mg/m2 (lapatinib) and 280mg/m2 (placebo). Lapatinib combined with CRT was well-tolerated. Grade 3/4 toxicities during CRT were balanced between arms, with the exception of an excess of grade 3 diarrhoea (6% versus 0%) and rash (9% versus 3%) and two grade 4 cardiac events in the lapatinib arm. CRR at 6 months post-CRT was 53% with lapatinib versus 36% with placebo in the intent-to-treat population. The progression-free survival (PFS) and overall survival rates at 18 months were 55% versus 41% and 68% versus 57% for the lapatinib and placebo arms, respectively. The difference between study arms was greatest in p16-negative disease (median PFS >20.4 months [lapatinib] versus 10.9 [placebo]). Conclusion Lapatinib combined with CRT is well-tolerated with numeric increases in CRR at 6 months post-CRT and median PFS in p16-negative disease.