BACKGROUND: We evaluated the prevalence of thyroid cancer in patients with cold thyroid nodules and the impact of sex, age, hardness of nodule, and multinodularity as factors able to predict the probability of malignancy in patients with nodular thyroid diseases. METHODS: We examined a 728 patients who visited Seoul National University Hospital with one or more cold thyroid nodules between Jan. 1996 and Dec. 1997. After clinical evaluations including medical history, physical examinations(size, hardness and multiplicity of nodule), fine needle aspiration biopsies and cytologic examinations were carried out. RESULTS: Among the 728 cases, 76 cases(10.4%) were diagnosed as cancer and 602 cases (82.6%) were diagnosed as benign nodule. The prevalence of cancer was significantly lower in female patients with cold nodules(9.4%, 62/662) than in males(17.5%, 11/63)(p=0.041). Age was an important factor in both sexes. The proportion of nodules that were malignant was lower in patients of 20-60 years old(8,9%, 56/632) than patients younger than 20 years old(1S.1%, 2/11) or older than 60 years old(18.3%, 15/82)(p=0.019). The prevalence of cancer was significantly higher in hard nodules(36.3%, 41/113) than firm(5.2%, 30/574) or soft nodules(5.3%, 2/38)(p= 0.001). There was no size difference between malignant(25.2 +- 13.7mm) and benign nodules(25.3 +- 8.9mm)(p=0.9425). The prevalence of thyroid cancer in solitary nodule(10.6%, 63/593) was not different from that in multiple nodules(7.6%, 10/132)(p=0.293). CONCLUSION: Our data suggest that thyroid nodules of the patients who are younger than 20 years old or older than 60 years old, male, as well as hard nodule require more careful evaluation for the risk of thyroid malignancies.