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Year 1003 (MIII) was a common year starting on Friday (link will display the full calendar) of the Julian calendar.
- January 25 – Werner, Margrave of the Nordmark, succeeds Lothair.
- February 9 – Boleslaus III of Bohemia is restored to authority, with armed support from Bolesław I the Brave of Poland. In the following months, Boleslav's brothers Jaromir and Oldrich flee to Germany, and place themselves under the protection of Henry II, Holy Roman Emperor, while Boleslaus III of Bohemia orders the massacre of his leading nobles at Vyšehrad.
- May 4 – Herman III, Duke of Swabia succeeds Herman II.
- Vladivoj of Bohemia succeeds Boleslaus III.
- Boleslaus IV succeeds Vladivoj of Bohemia.
- Bolesław I of Poland annexes parts of present-day Slovakia (possible date).
- Wilfred succeeds Abbot Oliva as count of Berga.
- Sweyn I of Denmark begins his first invasion of England.
- Humbert I becomes the first count of Savoy.
- King Robert II of France invades Burgundy, but fails.
- Robert II of France marries Constance of Arles.
- Boleslaus I of Poland becomes duke of Bohemia and Moravia.
- Stephen I of Hungary invades Transylvania, and organises the Diocese of Transylvania (possible date).
- Deutz Abbey is founded at Deutz, by Heribert of Cologne.
- Battle of Albesa: Troops of the Caliphate of Cordoba defeat the northern Christian armies.