There are too many uncertainties about Gregory's Registrum for any attempt to make a set of statistics reveal the pattern of his activities. In any case, it should be remembered that a very high proportion of his correspondence takes the form of ‘rescripts’, that is to say rulings made in reply to requests and queries originating with his correspondents. We do not know, even approximately, the extent of his correspondence. There are references to correspondence which is not among the surviving letters in the Registrum; and there may be many more of which there is no trace. The largest Book in the Registrum is Book IX, containing 240 letters for the second Indiction (September 598–August 599). This is the most completely preserved, as there is a large overlap of two of our surviving collections, R and C, in it: the number of letters preserved only in the collection ‘C’ is 145. The remaining letters are found in the collection ‘R’, the principal collection extending throughout the fourteen years and containing the largest number of letters; some are both in C and in R. R thus contains 95 of the 240 existing letters in Indiction II, approximately 40 per cent. If the same proportion were to hold throughout the Register, 60 per cent of the total would be missing. We cannot, of course, assume that the same proportion would hold, or that the 240 letters in Book IX represent the total correspondence for the year.