Purpose: Symptoms of urinary irritation, urgency, frequency and obstruction, known as lower urinary tract symptoms, are common in urological practice. However, little is known about the etiology or pathogenesis of lower urinary tract symptoms, especially the relative contributions of genetic and environmental factors to the development of these symptoms. We used a classic twin study design to examine the relative contributions of genetic and environmental factors to the occurrence of lower urinary tract symptoms in middle-aged men. Materials and Methods: Twins were members of the Vietnam Era Twin Registry. We used a mail survey to collect data on lower urinary tract symptoms using the I-PSS (International Prostate Symptom Score) instrument. Twin correlations and biometric modeling were used to determine the relative genetic and environmental contributions to variance in I-PSS total score and individual items. Results: Participants were 1,002 monozygotic and 580 dizygotic middle-aged male twin pairs (mean age 50.2 years, SD 3.0). Nearly 25% of the sample had an I-PSS greater than 8, indicating at least moderate lower urinary tract symptoms. The heritability of the total I-PSS was 37% (95% CI 32-42). Heritability estimates ranged from 21% for nocturia to 40% for straining, with moderate heritability (34% to 36%) for urinary frequency and urgency. Conclusions: Genetic factors provide a moderate contribution (20% to 40%) to lower urinary tract symptoms in middle-aged men, suggesting that environmental factors may also contribute substantially to lower urinary tract symptoms. Future research is needed to define specific genetic and environmental mechanisms that underlie the development of these symptoms and conditions associated with lower urinary tract symptoms.