Urodynamic investigation becomes increasingly important in the diagnosis of bladder outflow obstruction in patients with benign prostatic hyperplasia. To date, different methods for evaluation of the pressure-flow relationship and quantification of the grade of obstruction are available. Models for pressure-flow analysis are briefly explained. The variability of the parameters is investigated by evaluation of 75 patients in whom 2 sequential voidings during urodynamic investigation were analyzed. The results showed that in 87% of these patients individual maximum flow differences of first and second voidings were less than 2 ml. per second. Individual detrusor pressure at maximum flow differences were less than 15 cm. water in 80% of these patients, while in 80% the intra-individual variation of the pressure-flow results was less than 15 cm. water for the minimal voiding pressure parameters (minimal urethral opening detrusor pressure and urethral resistance factor). For the pressure-flow parameter that defines the theoretical urethral lumen during voiding, the variation was less than 1.5 mm.2 in 84% of the patients. Patients with larger intra-individual differences are discussed. We concluded that the observed, aforementioned differences can be regarded as an indication of normal intra-individual variability of voiding during urodynamic investigation. This intra-individual variability, however, seldom leads to a change in the clinical grade of bladder outflow obstruction. We conclude that investigators involved in therapeutic trials of benign prostatic hyperplasia must be aware of this intra-individual variability of micturition, since this variability is greater than the refined scale of the pressure-flow analysis models.