Whole-bladder-wall (WBW) photodynamic therapy (PDT) performed with 458 nm instead of 630 nm wavelength might be advantageous. On the basis of Monte Carlo (MC) computer simulations using in vitro bladder optical properties, these wavelengths show an equally strong integrating sphere effect, while haematoporphyrin derivatives can be excited equally efficiently and more easily with an Ar+ laser at 458 nm. To test this, fluence rates were measured at the walls of two piglet bladders during in vivo and in vitro WBW optical irradiations at 458, 488, 514 and 630 nm. In the in vitro experiment, a controlled amount of urine with known absorption coefficient at the irradiation wavelengths was introduced in the bladder cavity. The optical absorption and scattering coefficients and anisotropy factor of the tissue of both piglet bladders were determined in vitro with a double integrating sphere set-up. MC simulations, using the in vitro optical properties, agree only partly with the measured bladder wall fluence rates. In the in vitro experiment with saline in the bladder cavity, the fluence rate at the bladder wall is lowest for 514 nm irradiation and highest for 458 and 630 nm irradiation. In the in vivo experiment and the in vitro experiment with light absorbing urine in the bladder cavity, which mimics the clinical situation, irradiation at 458 nm wavelength resulted in the lowest fluence rate for a given optical power emitted. It cannot be completely ruled out that in an in vivo bladder the light absorption by haemoglobin further reduces the integrating sphere effect at wavelengths shorter than 630 nm. Thus, WBW PDT with red light (630 nm) is technically more advantageous than that with green light (514 nm) or blue light (488 and 458 nm) as this gives the strongest integrating sphere effect.