The observation of protoplanetary disks at the astronomical unit scale is one of the most exciting key programs for long baseline interferometers. Existing observations have questioned the models of circumstellar environments of young stars; but this is just the beginning. Here, to provide the reader with a sense of what can be done today in this field, actual observations of the pre-main sequence star FU Orionis with near-infrared interferometers IOTA and PTI are presented. Every step, from raw data reduction to final interpretation, is described. The limits of current observations and the promise of future contributions from the VLTI are discussed.