This study was designed to examine short-term blood pressure variability (BPV) in patients with different severity and forms of chronic medically treated hypertension. Power spectral analysis of BPV was performed from continuous finger blood pressure (Finapres) recordings. Ten patients with renovascular hypertension (RVHT), 34 with severe essential hypertension (SEHT) and 29 with mild essential hypertension (MEHT) as well as healthy age- and sex-matched control subjects were studied. The RVHT group was characterized by reduced low frequency (LF) power of both systolic and diastolic BPV (P =0.004 and P =0.003 respectively) when compared with the control group. There was also a tendency to lower total power of diastolic BPV (P =0.094). On the contrary, the SEHT group had increased total power of diastolic BPV (P =0.044). However, in the SEHT group, we found no differences in the LF and high frequency power of systolic and diastolic BPV when compared with controls. The MEHT group presented with lower LF power of systolic and diastolic BPV (P =0.028 and P =0.003 respectively) and, in addition, high frequency power of diastolic BPV was lower than in the control group (P =0.020). When the hypertensive groups were compared with each other, total power and LF power of diastolic BPV (P =0.043 and P =0.039 respectively) were lower in the RVHT group than in the SEHT group. In addition, total power of diastolic BPV was lower (P =0.030) in the MEHT group than in the SEHT group. No differences were observed in BPV between the RVHT and MEHT groups. The results show that BPV in hypertensive patients groups behaved differently. This suggests that both the aetiology and severity of hypertension have a significant influence on short-term BPV measured in laboratory conditions and that different control mechanisms are operating in these clinically distinctly different hypertension groups.