This is a randomised, double-blind, placebo-controlled trial to evaluate the efficacy of daily low-dose sildenafil (25mg) for the treatment of non-ulcer interstitial cystitis (IC) in women. Forty-eight women with clinical diagnosis of IC, from three centres, were equally randomised to the two groups and followed up every two weeks for three months. O’Leary-Sant IC symptom and problem indices, visual analog scale scores, and a micturition diary were recorded and compared. Significant improvement was noted from four weeks onwards and maximum improvement at three months, especially with voiding symptoms. Parameters in urodynamics – maximum cystometric capacity, first and strong desire to void volumes, improved and a transient improvement in pain – is seen at three months. The efficiency of treatment reached 62.5% and 42.5% could maintain the improvement at three months. The possible mechanism of action in IC could be reduction in the contraction sensitivity of the detrusor muscle, leading to its relaxation, improvement of microcirculation, and reabsorption of excessive potassium in the bladder cGMP dependent protein kinase G-RhoA / Rho kinase and nitric oxide or carbonic oxide signalling pathway. The authors recognise the limitations and need for randomised controlled trials in the future.