In this study, using the two-step truncated Fourier series with four harmonics, we analysed the diurnal blood pressure profile in 13 mild-moderate essential hypertensive patients during isradipine long-term therapy. Circadian parameters such as the amplitudes and phases of the four harmonics and the overall amplitude and phase were obtained from the model. The total duration of the study was 26 weeks. Ambulatory blood pressure was measured at 15-min, intervals using a Takeda TM 2420 device after 2 weeks of placebo and after 6 and 26 weeks of isradipine (5 mg daily) respectively. After 6 and 26 weeks therapy the blood pressure values showed a significant decrease, although the daily blood pressure curves obtained from Fourier analysis showed that the circadian rhythm was not altered by isradipine treatment. Both the night/day differences and the overall amplitude/acrophases were statistically significant at 0.6 and 26 weeks. According the nocturnal blood pressure fall, we found that long-term therapy with isradipine increased the number of patients with nocturnal blood pressure fall and reducing the early morning blood pressure rise. In conclusion, the two-step method Fourier analysis represents a novel and useful statistical approach to evaluate the presence of a significant diurnal blood pressure rhythm and to provide the circadian parameters of the 24-h blood pressure profile during pharmacological therapy.