Motivation: Photodynamic Therapy (PDT) with interstitial light delivery by multiple fibers for the treatment of large tissue volumes requires measurement of sensitizer distribution for dosimetric considerations. For stereotactic interstitial PDT of malignant glioma, for instance, a pre-irradiation comparison of the contrast enhancing tissue volume in MR-imaging with the photosensitized volume as assessed by fluorescence detection is desirable. For PDT of prostate cancer, the quantitative measurement of the selectivity of sensitizer uptake in cancer versus normal prostate parenchyma is important. Methods: It has previously been shown by others that the fluorescence intensity measured by a thin single optical fiber for excitation and detection is largely independent on optical parameters of the tissue that contains the fluorochrome. However, the investigators assumed similar values for excitation and emission wavelengths. This study concerned liquid phantom measurements (absorber: ink or hemoglobin, fluorochrome: Na-fluorescein) and Monte Carlo calculations, with extended conditions, where the absorption differs by a factor of 10 between excitation (426 nm) and emission (530 nm) wavelengths. The absorption coefficient (μa') was varied between 0.01 - 0.3 mm-1 (@ 426 nm), the effective scattering coefficient (μs') between 0.6 - 2.5 mm-1. A 200 μm and a 1000 μm core fiber were used. Results: Fluorescence intensity measured at 530 nm via a thin optical fiber (core diameter small compared to light penetration depth) depends minimally on optical tissue parameters. This result is valid for ink as absorber (μa identical at excitation and emission) as well as for hemoglobin (μa different). Fluorochrome concentration measurements seem possible with a 200 μm core fiber, but not with the 1000 μm core fiber.