As far as i know,

Islam strictly forbids the giving or taking of interest on any loan in any amount. Interest is defined as a set return on a loan, and as such implies a profit based on no risk or effort – a principle completely against Islamic ideals. Financial profit is a result of effort or risk (or both), and interest seeks to undermine the risk-based nature of trade.

http://www.islamonline.net/english/introdu...article01.shtml