\newcommand{\deflabel}[1]{\rm#1\hfill}%
\newenvironment{mylist}[1]%
{\begin{list}{}%
{\settowidth{\labelwidth}{\rm #1}%
\setlength{\leftmargin}{\labelwidth}%
\addtolength{\leftmargin}{\labelsep}%
\renewcommand{\makelabel}{\deflabel}}}%
{\end{list}}%