Fancy box around equation in align environment

If you don't mind loading TikZ, you may try

\documentclass{minimal} \usepackage{color} \usepackage{tikz} \usetikzlibrary{fit} \newcommand{\tikznode}[1]{ \tikz[remember picture,baseline=(#1.base)]{ \node(#1)[inner sep=0pt]{\strut}; } } \definecolor{myblue}{rgb}{0.8, 0.8, 1} \usepackage{amsmath,mathtools} \begin{document} Draw a box with border. \begin{align} \tikznode{1} \text{stuff} &= \text{other stuff}\tikznode{2} \end{align} \tikz[remember picture,overlay]{ \node[draw,fit=(1) (2)]{}; } Draw a box with a transparent overlay. \begin{align} \tikznode{3} \text{stuff} = \text{other stuff} \tikznode{4} \end{align} \tikz[remember picture,overlay]{ \node[fit=(3) (4),fill=blue,fill opacity=0.25]{}; } Draw both. \begin{align} \tikznode{5}\text{stuff} = \text{other stuff}\tikznode{6} \end{align} \tikz[remember picture,overlay]{ \node[draw,fit=(5) (6),fill=blue,fill opacity=0.25]{}; } It also works if you want to put the box around several lines \begin{align} \tikznode{7}E &= m_0\, c^2\notag \\ &=\frac{m}{\sqrt{1-v^2/c^2}\tikznode{9}}c^2\tikznode{8} \end{align}\tikz[remember picture,overlay]{ \node[draw,fit=(7) (8) (9),fill=blue,fill opacity=0.25]{};} As you see in this example, you need to place the tikznodes in the most extreme positions. \end{document} 

Of course, loading TikZ just for this might be overkill, but if you load it anyway and/or aim at even fancier styles, it might be worthwhile.