To determine the effect of phytotherapy (Serona repens) on the clinical progression in men with mild symptoms of bladder outlet obstruction (BOO). A total of 189 patients with mild symptoms of BOO, recruited from four different European clinics, were included in the analysis. Age, prostate specific antigen (PSA), international prostate symptom score (IPSS), quality of life (QOL), maximum urinary flow rate (Qmax) and total prostate and transitional zone volume were recorded. Clinical progression was defined as change from the mild-IPSS group into the moderate or severe group or the occurrence of urinary retention and need of surgery. Cumulative progression rate was 1, 7, 9 and 16% at 6, 12, 18 and 24 month, respectively, for the active group (Serona repens) as compared to 6, 13, 15 and 24% for the watchful waiting group. (P=0.03) significant improvements in the Qmax, IPSS and QOL were seen in the group receiving Serona repens. Serona repens significantly reduced the clinical progression rates in men with mild symptoms of BOO. It also led to improvements in urinary symptoms, QOL scores and urinary flow rates.