With the increasing use of thyroid ultrasound, the recognition of thyroid nodules in a large proportion of apparently healthy subjects has become common. Because also the papillary thyroid microcarcinomas (PTMC) are being increasingly discovered, it is important to ascertain whether PTMC may exhibit heterogenous clinical features, associated with different aggressiveness.We retrospectively examined 122 subjects [98 female (80.3%), and 24 male (19.7%)] with thyroid cancer to find potential clinical and pathological findings that could be predictive of clinically aggressive behavior.Twenty of the 31 patients with true incidental cancer (64.5%) in comparison to 20 of the 91 patients with non-incidental cancer (21.9%) had a diameter <10 mm, and this difference was statistically significant (p<0.0001). There was a statistically significant association between size and invasiveness because 19.3% of invasive cancers were <10 mm whereas 44.6% of non-invasive cancers were <10 mm (p=0.005). The relationship between incidental discovery and invasiveness was also evaluated, but the proportion of incidental invasive cancer (19.3%) was not significantly different from that of incidental non-invasive cancer (30.8%). In the multivariate analysis, only size <10 mm (odds ratio=0.35, p=0.013) and papillary vs other histotypes (odds ratio=0.35, p=0.04) were statistically significant protective factors against invasiveness.a) Incidentally discovered thyroid cancers are more frequently microcarcinomas; b) there appears to be no difference in terms of invasive behavior between incidental and non-incidental thyroid cancer; c) smaller tumor size emerges as a protective factor.