This paper reports a reanalysis of calcification rates of 328 Porites cores from the Great Barrier Reef from which previous workers have concluded that a 14% reduction in calcification rates has occurred between 1990 and 2005. In this reanalysis it is shown that the apparent reduction in the Porites spp. calcification rate in the last two decades is at least partly due to a combination of (a) ontogenetic effects (disregarded in the previous analysis), combined with a highly variable age distribution of the coral growth bands with time, and (b) a systematic data bias clearly evident in the last growth band of each core. When the outermost growth band in addition to bands which have record age less than 20 years was excluded from the analysis, the dramatic fall in calcification after 1990 was no longer evident.