Introduction: We present a new bibliometric indicator to measure journal specialisation over time, named the focus factor. This new indicator is based on bibliographic coupling and counts the percentage of re-citations given in subsequent years. Method: The applicability of the new indicator is demonstrated on a selection of general science journals and on a selection of medical journals. The reference lists of each journal are compared year by year, and the percentage of re-citations is calculated by dividing the number of re-citations with the total number of citations each year. Analysis: To validate re-citations as caused by specialisation, other possible causes were measured and correlated (obsolescence, journal self-citations and number of references). Results: The results indicate that the focus factor is capable of distinguishing between general and specialised journals and thus effectively measures the intended phenomenon (i.e., journal specialisation). Only weak correlations were found between journal re-citations and obsolescence, journal self-citations, and number of references. Conclusions: The focus factor successfully measures journal specialisation over time. Measures based on either simple citation analysis or bibliographic coupling are found to be close relatives. Measures based on journal self-citation are found to be only weakly correlated with the focus factor. Measures based on co-citation analysis remain to be studied and compared.