The Indonesian region was divided in 23 seismogenic sources and the interevent times of strong shallow mainshocks were determined and used for each source in an attempt at long-term forecasting. The following relations have been obtained: log Tt=0.15Mmin+0.34Mp-0.34 log mo+6.54 and Mf=0.92Mmin-0.34Mp+0.53 log mo-10.60, where Tt is the interevent time, measured in years, Mmin the surface wave magnitude of the smallest mainshock considered, Mp the magnitude of the preceding mainshock, Mf the magnitude of the following mainshock, and mo the moment rate in each source per year. On the basis of these relations and taking into account the time of occurrence and the magnitude of the last mainshock in each seismogenic source, time dependent conditional probabilities for the occurrence of the next large (Ms≥7.0) shallow mainshocks during the next 10 years as well as the magnitudes of the expected mainshocks are determined. ¿ American Geophysical Union 1994