Aims : To compare cell proliferation markers, minichromosome maintenance protein 2 (MCM2) and Ki67, in minimally invasive follicular carcinoma (MIFC) and follicular adenoma (FA) of the thyroid and among MIFCs with different diagnostic criteria.Methods and results : Twenty‐two MIFCs and 20 FAs were immunohistochemically stained for MCM2 and Ki67. The MIFCs were subdivided into six Group 1 tumours with both capsular and vascular invasions, seven Group 2 tumours with vascular invasion only and nine Group 3 tumours with capsular invasion only. The MCM2 and Ki67 indices were calculated, counting more than 1000 tumour cells in the most frequently positive areas. In total and Groups 1–3 MIFCs and in FAs, the average MCM2 index was 26.7 ± 11.0, 28.4 ± 8.6, 26.3 ± 14.8, 25.9 ± 8.4 and 10.7 ± 4.5, respectively, whereas the average Ki67 index was 2.07 ± 1.65, 1.93 ± 2.02, 2.49 ±1.38, 1.84 ± 1.5 and 1.78 ± 0.92, respectively. There was a significant difference in the MCM2 index, but not in the Ki67 index, between each category of MIFCs and FA (P < 0.01). However, neither the MCM2 index nor the Ki67 index showed a statistically significant difference among the subgroups of MIFC.Conclusions : MCM2, but not Ki67, is a helpful marker for differentiating MIFC from FA. The tumour cell proliferative activity supports the histological criteria based on diagnosing MIFC by either capsular or vascular invasion only.