The objective of this study was to assess the intra- and inter-investigator variation in the analysis of pressure-flow studies that were performed in men with lower urinary tract symptoms. Two hospitals were involved in this study. In each hospital 100 PFS were selected. Photocopies of printouts of all PFS were analyzed manually by six experienced investigators, including determination of P(detQmax) and Q(max). Afterward, all 200 PFS were analyzed again in a different order. For each P(detQmax) and accompanying Q(max) the AG-number was calculated. With these AG numbers, the intra-investigator SD, the inter-investigator SD and the intra- and inter-investigator SD combined were calculated. The intra- and inter-investigator SD combined was 10.7. This implies that if one investigator analyzes a PFS once and determines an AG number of 40, another investigator may determine an AG number between 40 +/- 2. 77*10.7 = 10-70, using a 95% confidence interval. The inter-investigator SD was 10.0 and the intra-investigator SD was 3.7. The reproducibility of the manual analysis of urodynamic studies is moderate owing to a considerable intra- and inter- investigator variation. This is mostly caused by the substantial intra-investigator variation.