Populations of the common hippopotamus Hippopotamus amphibius in Mozambique were surveyed in 2010 during a national survey of the crocodile Crocodylus niloticus. Numbers of hippos seen during aerial counts along major rivers and lake shores were corrected to allow for undercounting of groups and these data were supplemented with the results of other recent surveys. There are now estimated to be c. 3,000 hippos in Mozambique and c. 50% of these live in Lake Cabora Bassa or the Zambezi River. The national total is much lower than the figure of 16,000-20,500 hippos estimated in 1986, used for the latest (v. 2012.1) IUCN Red List. The 1986 total included an estimated 10,000-12,000 in Marromeu Complex, an area that includes the southern Zambezi delta. We review the results of past surveys and find that the number of hippos in Marromeu Complex in 1986 was probably three times fewer than estimated. Although the number of hippos in this area declined markedly during the 1980s we believe that the 1986 overestimate of hippos in Marromeu Complex is an error that has been perpetuated for 25 years. Particular care should be taken when Red List assessments roll-over old and unsupported estimates of numbers. Even if an old estimate was accurate there comes a time when it should not simply be rolled-over. The 2007 IUCN Species Survival Commission's African Elephant Status Report provides a model for future assessments of the status of the common hippopotamus, categorizing the numbers of a species according to the type of survey, its reliability, and how long ago it was conducted.