How to put white space under header in theorem environment

Ok, I am pretty new with latex so maybe this is a basic question. I am writing in theorem environment and I want to produce only white space under my theorem header, with the text continuing right after it, but the text goes automatically under the header. any help?

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Welcome to TeX.sx! Please add a minimal working example (MWE) that illustrates your problem. Note that there are packages which modify the behavioir of theorem environments. –  lockstep Oct 18 '12 at 9:24
Don't. The theorem heading and the text in italics is more than sufficient to give emphasis to the statement. If the statement is long, you'll get a very unbalanced page; moreover "Lemma" and "Theorem" have different widths, which is another problem to overcome. –  egreg Oct 18 '12 at 11:10
I'm not sure if I understand your objective fully. If you want to have a linebreak between the theorem's header and body (but have the body's text start flush-left), you could employ the ntheorem package and specify \theoremstyle{break}. –  Mico Oct 18 '12 at 12:16

Assuming you use amsthm, you can define new theorem style that issues the endline after the theorem heading:

\documentclass{article}

\usepackage{amsthm}

\newtheoremstyle{plain2}
{3pt}% Space above
{3pt}% Space below
{}% Body font
{}% Indent amount
{\bfseries}% Theorem head font
{\\*[3pt]}% Punctuation after theorem head % HERE CHANGE THE SPACING
{.5em}% Space after theorem head
{}% Theorem head spec (can be left empty, meaning normal')

\theoremstyle{plain2}

\newtheorem{theorem}{Theorem}

\begin{document}

\begin{theorem}[My theorem]
This is my theorem.
\end{theorem}

\end{document}
`
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