Background The present study examined the role of aldosterone in left ventricular hypertrophy (LVH) and geometry in patients with untreated essential hypertension (EHT), and investigated the contribution of myocardial fibrosis to the process of LVH. Methods and Results The relationship of the plasma aldosterone concentration (PAC) to LVH and left ventricular (LV) geometry was investigated in 57 consecutive patients with untreated EHT. PAC correlated with both LV mass index (LVMI: r=0.46, p=0.0004) and relative wall thickness (RWT: r=0.33, p=0.013). In patients with LVH (LVMI ≥125 g/m²), the serum concentration of procollagen type III amino-terminal peptide (PIIINP), a marker of myocardial fibrosis, correlated with RWT (r=0.46, p=0.029). These patients were divided into 2 groups: concentric hypertrophy (CH) with RWT ≥0.44, and eccentric hypertrophy (EH) with RWT <0.44. The serum PIIINP concentration was significantly higher in the CH group than in the EH group (0.52±0.02 ng/ml vs 0.44±0.03 ng/ml, respectively; p<0.05). Conclusions Aldosterone may be involved in LVH and LV geometry, particularly in the development of CH. Myocardial fibrosis seems more strongly involved in the hypertrophic geometry of CH than with EH. (Circ J 2007; 71: 716 - 721)