Proinsulin has generally been regarded as an inert precursor to insulin. However, over the past few years, proinsulin has established itself as a useful research tool for understanding how cells synthesize and secrete peptide hormones. Last year, proinsulin attracted renewed interest for its role as the precursor to C peptide, which may prove useful in the treatment of patients suffering from insulin-dependent diabetes mellitus. This mini-review focuses on three aspects of proinsulin, each of which attracted attention in 1997. These three aspects illustrate how this peptide hormone precursor may yet prove to be more important than its primary role as a prohormone, with only one bioactive product, would suggest.