We report results of in-vivo light dosimetry of light fluence (rate) in human prostate during photodynamic therapy (PDT). Measurements were made in-vivo at the treatment wavelength (732nm) in 15 patients in three to four quadrants using isotropic detectors placed inside catheters inserted into the prostate. The catheter positions are determined using a transrectal ultrasound (TRUS) unit attached to a rigid template with 0.5-cm resolution. Cylindrical diffusing fibers with various lengths are introduced into the catheters to cover the entire prostate gland. For the last four patients, distributions of light fluence rate along catheters were also measured using a computer controlled step motor system to move multiple detectors to different distances (with 0.1 mm resolution). To predict the light fluence rate distribution, a kernel-based model was used to calculate light fluence rate using either (a) the mean optical properties (assuming homogeneous optical properties) for all patients or (b) using distributions of optical properties measured for latter patients. Standard deviations observed between the calculations and measurements were 56% and 34% for (a) and (b), respectively. The study shows that due to heterogeneity of optical properties significant variations of light fluence rate were observed both intra and inter prostates. However, if one assume a mean optical properties (µa = 0.3 cm-1, µs' = 14 cm-1), one can predict the light fluence rate to within a maximum error 200% for 80% of the cases and a mean error of 105%. To improve the prediction of light fluence rate further would require determination of distribution of optical properties.