This should not be the case by default with the provided line, but perhaps some other package or command included in your preamble (or document) that modifies the \baselineskip length. For example, the following MWE:
\documentclass[a4paper,10pt]{article}
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\begin{document}
\lipsum[1-4]
\end{document}
produces as-expected line spacing given by a 12pt (or \baselineskip) as a result of specifying a 10pt option in the document class.
Regardless, if you do want to change line spacing, it is preferred to use the setspace package. It provides commands like \singlespacing, \onehalfspacing, \doublespacing (all of which are toggles/switches, with singlespace[*], onehalfspace and doublespace environment counterparts) and \setstretch{<num>}. The file setspace.sty includes the documentation.
onehalfspacingortwohalfspacingor the command\onehalfspacingor\twohalfspacingat the preamble? – Schweinebacke Nov 21 '11 at 18:17\documentclassand the appropriate packages. Personally, I have often solved my own problems in the process of reducing the amount of code actually required to reproduce the problem. – Peter Grill Nov 21 '11 at 21:01