Fluence rates were measured in vivo at a piglet bladder wall during whole bladder wall (WBW) light irradiation at 458, 488, 514, 532, and 630 nm wavelengths. Bladder optical properties, the absorption-, scattering-, and anisotropy coefficient, were determined in vitro at these wavelengths using a double integrating sphere set-up. Monte Carlo (MC) computer simulations for WBW photodynamic therapy (PDT) were performed in a spherical geometry representing the bladder. The in vivo measured fluence multiplication factor ((beta) ) decreases from approximately equals 5 at 630 nm to approximately equals 1.5 at 458 nm. The simulated (beta) values, using the in vitro optical properties and non-absorbing (saline) bladder contents, are consistently larger with a minimum at 514/532 nm and a maximum at 458 and 630 nm. Simulations with slightly light absorbing bladder contents show that the inevitable urine in the cavity can at least partly be responsible for the lower in vivo values. Whereas the MC simulations use an in vitro absorption coefficient, the in vivo observed phenomenon might be attributed to additional light absorption by hemoglobin in the bladder tissue. Thus, WBW-PDT with red light is technically more advantageous than with green or blue light as this gives the strongest integrating sphere effect.