Intralipid is a light scattering medium used in photodynamic therapy (PDT) in both phantom studies and to obtain optimum light distributions from clinical light delivery systems. Light delivery to a target is one variable in PDT over which we have direct control, and should be as accurate as possible. Variations in the scattering ability of Intralipid from batch to batch will lead to an unpredictable light distribution and undesirable PDT effects. Furthermore, 10% and 20% stock solutions, when diluted to similar final concentrations, have different scattering abilities which will lead to inaccuracies in studies measuring scattering coefficients and imprecision in light distributions from light delivery systems. Batch to batch variations of similar stock solutions should lead to minimal differences in outputs from light delivery systems. A simple and quick assay system is described to define the concentration of Intralipid based on its light scattering ability, which enables testing of solutions prior to use. In the clinical setting where accurate light distribution is required, we suggest that only one stock solution (e.g. 20%) is used and that the final dilution scattering ability is checked using this assay system.