BACKGROUND: UK armed forces personnel who took part in the 1991 Gulf war experienced an increase in symptomatic ill health, colloquially known as Gulf war syndrome. Speculation about an Iraq war syndrome has already started. METHODS: We compared the health of male regular UK armed forces personnel deployed to Iraq during the 2003 war (n=3642) with that of their colleagues who were not deployed (n=4295), and compared these findings with those from our previous survey after the 1991 war. Data were obtained by questionnaire. FINDINGS: Graphs comparing frequencies of 50 non-specific symptoms in the past month in deployed and non-deployed groups did not show an increase in prevalence of symptoms equivalent to that observed after the Gulf war. For the Iraq war survey, odds ratios (ORs) for self-reported symptoms ranged from 0.8 to 1.3. Five symptoms were significantly increased, and two decreased, in deployed individuals, whereas prevalence greatly increased for all symptoms in the Gulf war study (ORs 1.9-3.9). Fatigue was not increased after the 2003 Iraq war (OR 1.08; 95% CI 0.98-1.19) but was greatly increased after the 1991 Gulf war (3.39; 3.00-3.83). Personnel deployed to the Gulf war were more likely (2.00, 1.70-2.35) than those not deployed to report their health as fair or poor; no such effect was found for the Iraq war (0.94, 0.82-1.09). INTERPRETATION: Increases in common symptoms in the 2003 Iraq war group were slight, and no pattern suggestive of a new syndrome was present. We consider several explanations for these differences.