OBJECTIVES: To evaluate the efficacy and safety of the phosphodiesterase type 5 inhibitor, UK-369,003 modified release (MR), for the treatment of storage lower urinary tract symptoms (LUTS) in men with and without erectile dysfunction (ED). PATIENTS AND METHODS: This was a multicentre, double-blind, placebo-controlled, parallel-group study conducted across 50 centres in North and South America, Europe and Australia. In all, 310 men aged ≥ 18 years with a clinical diagnosis of overactive bladder (OAB; voiding frequency ≥ 8 times/24 h, urgency episode frequency once or more per 24 h and a mean voided volume of <300 mL) and maximum urinary flow rate of >5 mL/s in a voided volume of >150 mL were stratified into two groups (with or without ED) and randomized to one of five treatment groups (10, 25, 50 or 100 mg UK-369,003; or placebo once a day) for 12 weeks. The primary study endpoints were those derived from the bladder diary that recorded the number of voluntary urinary voids, volume of urine per void, leaks and urgency episodes over a 72-h period, before baseline and again at 2, 4 and 12 weeks. Secondary efficacy measures included the International Prostate Symptom Score (total and storage and voiding subscores), International Index of Erectile Function-Erectile Function domain (IIEF-EF), questions 5 and 6 of the Quality of Erection Questionnaire (QEQ), the Overactive Bladder Questionnaire Short Form, the Patient Perception of Bladder Condition, the International Consultation on Incontinence Questionnaire-Male LUTS, and the patient-reported treatment impact questionnaire. RESULTS: Overall, there were no clinically relevant treatment differences in voiding frequency, mean voided volume, urgency episode frequency, or nocturia frequency for any dose of UK-369,003 MR compared with placebo. In the subset of patients with ED there were improvements in the IIEF-EF and QEQ scores in all UK-369,003 treatment groups compared with placebo. CONCLUSIONS: These data provide no evidence of efficacy for UK-369,003 in the treatment of storage LUTS in men (based on classic OAB eligibility criteria). However, although the endpoints on these classic OAB efficacy variables were negative, there is evidence to suggest a greater preference, satisfaction and willingness to use UK-369,003 again for all treatment groups compared with placebo.