We evaluated the prognostic significance of overexpression of cyclin D1 in 47 patients with surgically resected squamous cell carcinomas of the head and neck. Overexpression of cyclin D1 was detected immunohistochemically using an affinity-purified polyclonal antibody directed against the carboxyl-terminal part of the cyclin D1 protein, applied to formalinfixed, paraffin-embedded tissue sections. Overexpression of cyclin D1 was found in 30 of 47 head and neck squamous cell carcinoma (HNSCC) cases and was associated with a more rapid and frequent recurrence of disease (P = 0.027). There was a 5-year disease-free interval of 47% for HNSCC patients with a strong overexpression of cyclin D1 and of 80% for cyclin D1-negative HNSCC patients. Overexpression of cyclin D1 was also associated with a shortened overall survival of these patients (P = 0.0095), with a 5-year survival of 60% for the cyclin D1 strongly positive cases and of 83% for cyclin D1-negative cases. Overexpression of cyclin D1 appears to indicate poor prognosis in operable HNSCC.