Aminophylline was formulated as small spherical cores for subsequent coating in an attempt to develop a competitor microencapsulated product to the commercial available sustained-release tablet, Phyllocontin. Optimum spherical crystallization conditions yielded cores of loosely adhering crystals of active, with highly irregular surface morphology and poor mechanical strength during pan coating. Aqueous spheronization yielded satisfactory cores in high yield when microcrystalline cellulose and liquid paraffin were used. However, application of large amounts of controlled-release coatings based on Eudragit RL and RS failed to produce a product with retarded drug dissolution comparable to the commercial product. Drug loaded non-pareils were easily formed, but required application of about 20 per cent Eudragit RL/RS coating to achieve adequate prolonged-release properties. Application of 10 per cent hydrogenated castor oil/ethylcellulose based coating gave acceptable in vitro release only if the microcapsules formed were tableted and annealed. All products investigated rapidly discoloured during storage and none were considered to represent a realistic alternative to tableting technology for the production of a sustained-release oral dosage form of aminophylline.