A 26-year-old man developed a fluctuating swelling of the forehead after twice being treated with antibiotics because of persistent sinusitis complaints. This swelling, a subperiostal abcess diagnosed as 'Pott's puffy tumor', is a complication of a frontal sinusitis, and may lead to serious neurological sequelae. The patient recovered after surgical drainage, ethmoidectomy and intravenous antibiotic treatment; no ablation of affected bone was necessary.