The Thornthwaite water balance technique is used to predict monthly stream runoff from Rio Macho basin in Costa Rica. The method was tested by comparing observed and predicted runoff over a 15 water-year period. The study shows that this method provides mean annual and monthly estimates in close agreement with measured values. Generally these mean estimated values fall between the 90% confidence intervals for the measured runoff. These results indicate that the Thornthwaite method can be satisfactorily applied to estimate mean monthly streamflow in the uplands of Costa Rica.