The effect of supplementary lighting (SL) on dry matter production of greenhouse crops is predictable with ALSIM, a new crop growth model based on SUCROS87. The light utilization efficiency (LUE), defined as daily dry matter production divided by the daily photosynthetic photon flux is a parameter for evaluating the quantitative effect of light on dry matter production. In winter in the Netherlands the LUE is raised by SL, especially by lighting in the natural dark hours. In summer there is hardly influence of SL on the LUE for the simulated growth of a fictitious crop. In winter the leaf area index (LAI) combined with biomass has a great influence on the LUE. Without SL the LUE is already negative at LAI above 2.3, due to a higher respiration than assimilation. SL makes it possible to produce dry matter at higher LAI values. Supplying CO2 in the greenhouse can partially compensate a light shortage. A combination of CO2 and SL can raise the LUE more than the sum of the separate effects. ALSIM is valuable for predicting growth in many situations for many crops if specific crop parameter values are available.