A highly localized incidence of skeletal malformations (tumors) in the scleractinian corals Platygyra pini and P. sinensis on an inshore fringing reef at Cockle Bay, Magnetic Island within the Great Barrier Reef province is reported. These tumors are typified by a localized area of increased growth rate resulting in roughly circular protuberances extending up to 4.5 cm above the colony's surface. In both species, similar proportions of their populations carried tumors (24.1 % in P. pini and 18.7 % in P. sinensis). Larger colonies (>80 cm in diameter) are at least 7 times more likely to possess tumors than smaller colonies (<40 cm in diameter). X-radiographs of the skeletal malformations indicate a point of origin, presumably from a single budded polyp with subsequent, localized, accelerated growth. The mean radial growth rate of the tumorous area was 29 % greater than that of the surrounding normal regions. In contrast to the normal tissue, the tumorous tissue exhibited proliferation of cells, atrophied gastrodermal cells and mesenterial filaments which were larger and disordered in structure. The environmental conditions at Cockle Bay are relatively extreme with high turbidity, periodic exposure of the reef flat, abrupt changes in salinity during the wet season and mechanical damage to corals caused by unpredictable cyclonic storms. It is suggested that a combination of environmental stresses coupled with an injury inflicted on the corals are possible stimuli that initiate the development of these abnormal growth through either bacterial attack or the development of an aberrant polyp during tissue repair.