CSS line problem?

How do I fix the line height in css. i.e. I have a header, under it (with very litle space between the lines) I want a text. The header is a H1 and the text under is unspecified, but the font size differ.

If you don't want to define the text then use or whatever value you want, plus I think you can use em (not sure though I must admit I don't use line-height much) but I would add it in a span or something because you will alter the website all at once and it will mess with the site layout.

Yep, it is line-height: you use px, em, percentage - usually use px, although I don't think it will take the fractions of a pixel though-

your right it will only take whole numbers for px but just to note 1.5px will render just as 2px (but I think the new iphone 4 my allow 4.5px because it works differently to others but to be honest it makes very little difference)

yes. better you make own class for the text and use span for it, so it won't mess up the whole lines in page's content.
btw, besides line-height, you can also use padding-top for the text, or padding rule for the header..