To explore the relationship between uroflow variables and lower urinary tract symptoms (LUTS): to define performance statistics (sensitivity, specificity, positive and negative predictive values) for maximum urinary flow rate (Qmax) with respect to bladder outlet obstruction (BOO) at various threshold values; and to investigate the diagnostic value of low-volume voids. The study comprised 1271 men aged between 45 and 88 years recruited from 12 centres in Europe, Australia, Canada, Taiwan and Japan over a 2-year period. Symptom questionnaires, voiding diaries, uroflowmetry and pressure-flow data were recorded. The relationship between uroflow variables and symptoms, Qmax and BOO, and the diagnostic performance of low volume voids were analysed. The relationship between symptoms and uroflow variables was poor. The mean difference between home-recorded and clinic-recorded voided volumes was -48 mL. Qmax was significantly lower in those with BOO (9.7 mL/s for void 1) than in those with no obstruction (12.6mL/s; P<0.001) and Qmax was negatively correlated with obstruction grade (Spearman's correlation coefficient -0.3, P<0.001), even when controlling for the negative correlation between age and Qmax (Spearman's partial correlation coefficient -0.29, P<0.001). A threshold value of Qmax of 10 mL/s had a specificity of 70%, a positive predictive value (PPV) of 70% and a sensitivity of 47% for BOO. The specificity using a threshold Qmax of 15 mL/s was 38%, the PPV 67% and the sensitivity 82%. Those voiding <150 mL (n=225) had a 72% chance of BOO (overall prevalence of BOO 60%). In those voiding >150 mL the likelihood of BOO was 56%. The addition of a specific threshold of 10 mL/s to these higher volume voiders improved the PPV for BOO to 69%. While uroflowmetry cannot replace pressure-flow studies in the diagnosis of BOO. it can provide a valuable improvement over symptoms alone in the diagnosis of the cause of lower urinary tract dysfunction in men presenting with LUTS. This study provides performance statistics for Qmax with respect to BOO: such statistics may be used to define more accurately the presence or absence of BOO in men presenting with LUTS, so avoiding the need for formal pressure-flow studies in everyday clinical practice, while improving the likelihood of a successful outcome from prostatectomy. This study also shows that low-volume uroflowmetry can provide useful diagnostic information and that, as such, the data from such voids should not be discarded.