His bundle electrograms of 40 patients developing Wenckebach block during atrial pacing and four with spontaneous Wenckebach block above the His were reviewed to determine the frequency of classical Wenckebach periodicity. Thirty patients had 143 Wenckebach cycles that were suitable for analysis. Cycles were evaluated for the following features: 1) the first A-H interval as the shortest, 2) the first R-R interval as the longest, 3) the last R-R interval as the shortest, 4) a progressive diminution of the increment of A-H interval prolongation, 5) a progressive diminution of the R-R interval and 6) the R-R interval containing the nonconducted A wave being equal to twice the A-A interval less the sum of the increments of A-H prolongation. Wenckebach cycles that occurred during atrial pacing were not significantly different from those that occurred spontaneously. Fifteen per cent of all cycles met all six criteria- 14% had five, 6% had four; 17% had three; 20% had two; 27% had one; and 1% had none. Short cycles were the most likely to show typical Wenckebach periodicity: 56% of the cycles with conduction ratios of 4:3, 28% with 5:4 and 4% with 6:5 met at least five criteria, whereas none of the 22 cycles having ratios 7:6 or greater had more than three features. The first A-H interval as the shortest was the most common feature occurring in 98% of cycles, whereas the features of a progressive diminution of the increments of the A-H interval prolongation or the progressive diminution of the R-R interval were the least common, occurring in 35% of cycles. These findings indicate, therefore, that classical Wenckebach periodicity is uncommon, especially when conduction ratios are 5:4 or greater. The implications of these observations and the suggested mechanisms are discussed and literature reviewed.