Summary Aims To demonstrate the correlation between morphological and proliferation parameters with the biological behaviour of adrenocortical tumours (ACTs) in different age groups. Methods Seventy-six cases of ACT were evaluated comparing tumours of children (≤15 years, n =24) and adults (>15 years, n =52) regarding morphological features and proliferation rates. These parameters were also compared in clinically benign (CB) and clinically malignant (CM) tumours. Tumours were analysed according to Weiss’ histological parameters. Proliferation rates were assessed by MIB-1 labelling index and silver binding nucleolar organiser regions (AgNORs) were quantified by image analysis. Results ACTs in children presented higher proliferative activity than in adults. In adult's ACTs, MIB-1 labelling index (cut-off, 10%) (P =0.037) and the AgNOR pattern (P <0.001) had influence over the disease-free survival (DFS). The AgNOR pattern type I was associated with CB-ACT and the AgNOR pattern type II with CM-ACT. The AgNOR pattern type III was characteristic of children's ACTs and type IV was characteristic of non-neoplasic adrenal glands. Conclusion ACTs in children had a different morphological spectrum and higher proliferative activity than those tumours in adults, which should be always analysed as a separate group. In adults, our findings reinforce that the morphological criteria proposed by Weiss are an efficient way of differentiating CB-ACT from CM-ACT.