Irregular protein secondary structures are believed to be important structural domains involved in molecular recognition processes between proteins, in interactions between peptide substrates and receptors, and in protein folding. In these respects tight turns are being studied in detail. They also represent template structures for the design of new molecules such as drugs, pesticides, or antigens. Isolated alpha-turns, not participating in alpha-helical structures, have received little attention due to the overwhelming presence of other types of tight turns in peptide and protein structures. The growing number of protein X-ray structures allowed us to undertake a systematic search into the Protein Data Bank of this uncharacterized protein secondary structure. A classification of isolated alpha-turns into different types, based on conformational similarity, is reported here. A preliminary analysis on the occurrence of some particular amino acids in certain positions of the turned structure is also presented.