For the best part of a decade Constantin Gruiescu was one of the best flyweight boxers in Romania. Gruiescu won his first title at the Romanian Amateur Boxing Championships in 1967 and retained it the following year. He finished as the runner-up in 1969 before going on to win back-to-back titles in 1970 and 1971. By this time he had moved up to the international circuit where he won bronze at the European Championships in 1971 in Madrid. He made his Olympic début the following year at the München Games where he lost to Douglas Rodríguez. Gruiescu’s finest moment came at the 1973 European Championships when he won gold with victory against Vicente Rodríguez of Spain. He then won bronze at the 1974 World Championships, before winning another European medal with silver in 1975. He competed at his second Olympics, this time at the 1976 Montréal Games, where he lost to the eventual gold medallist Leo Randolph.