In a prospective manner we sought to determine the cause of lower urinary tract symptoms in young men and whether noninvasive testing and symptoms scores are useful in deciding which patients to evaluate with videourodynamics. We evaluated 85 men 18 to 45 years old with lower urinary tract symptoms. Patients with a history of known neurological disease or urethral stricture were excluded from the study. Patients were evaluated with the American Urological Association (AUA) symptom index, noninvasive uroflowmetry post-void residual and videourodynamics, and classified by specific urodynamic diagnoses. Noninvasive uroflowmetry (normal versus abnormal), post-void residual and AUA symptom index (total, voiding and storage scores) were evaluated as predictors of urodynamic abnormalities. Mean patient age was 35.1 (range 18 to 45) and mean symptom duration was 53.8 months. Mean AUA scores were total 19.3, voiding 10.8 and storage 8.5. Videourodynamic diagnoses were primary bladder neck obstruction in 40 (47%) cases, dysfunctional voiding in 12 (14%), impaired contractility in 8 (9%), sensory urgency in 7 (8%), detrusor instability alone in 5 (6%), detrusor instability and impaired contractility in 1 (1%), external detrusor-sphincter dyssynergia in 1 (1%) and normal in 5 (6%). Of these patients, 9 could not void during urodynamics and in 6 (7%) no urodynamic diagnosis was made. Videourodynamics were not considered helpful in patients with a normal or nondiagnostic study or sensory urgency only (group 1) but were helpful or diagnostic in the remaining patients (group 2). Only 5 of 18 patients (28%) in group 1 had an abnormal uroflow compared to 56 of 67 (84%) in group 2 (p <0.0001). Mean post-void residual volumes were not different between the 2 groups (40.3 versus 40.0 ml.). Mean total and storage symptom scores were also not different between the 2 groups but voiding scores were significantly higher in group 2 (11.5 versus 8.3, p <0.03). Lower urinary tract symptoms in young men have a variety of underlying causes. Videourodynamics is an extremely helpful diagnostic test especially in men with abnormal uroflow and high voiding scores.