The current paper is concerned with exploring information contained in a series of undisclosed orders that are submitted by the same broker, using this information to estimate the volume contained in the current undisclosed order, and further investigating the trading patterns followed by stockbrokers in the use of undisclosed orders. In an ARMA framework, the estimation results suggest that the information revealed in past-executed undisclosed orders of a stockbroker is explanatory to the volume of the current undisclosed order submitted by the same broker. As a supplement to the current literature relating to the package-trading patterns detected in large disclosed orders, the current study finds that stockbrokers follow the same pattern in the use of undisclosed orders. A practical application of this method is to use it for the prediction of the volume enclosed in a given undisclosed order.