In a case-control, serially matched study, 70 patients with thyroid cancer, 55 with benign thyroid disease and 71 controls were interviewed in regard to a variety of socioeconomic, social and dietary characteristics. Statistical analysis revealed a strikingly negative (p less than 0.05) association between benign and malignant thyroid disease and consumption of coffee. After adjustment for possible confounding variables, the association remained statistically significant. The mechanism by which coffee consumption may play a protective role against development of benign or malignant thyroid neoplasms may be the stimulatory effect of caffeine on the intracellular cyclic AMP production, which is known to inhibit cell growth.