Background Giardia lamblia is a common cause of gastroenteritis worldwide, but there is limited knowledge about the long-term complications.

Objective To estimate the relative risk of irritable bowel syndrome (IBS) and chronic fatigue 3 years after acute giardiasis.

Design Controlled historic cohort study with 3 years' follow-up. Data collected by mailed questionnaire.

Setting Waterborne outbreak of giardiasis in the city of Bergen, Norway.

Participants 817 patients exposed to Giardia lamblia infection verified by detection of cysts in stool samples and 1128 matched controls.

Main outcome measures IBS and chronic fatigue.

Results The prevalence of IBS in the exposed group was 46.1%, compared with 14.0% in the control group, and the adjusted RR=3.4 (95% CI 2.9 to 3.8). Chronic fatigue was reported by 46.1% of the exposed group and 12.0% of the controls, the adjusted RR was 4.0 (95% CI 3.5 to 4.5). IBS and chronic fatigue were associated and the RR for the exposed group of having a combination of the two outcomes was 6.8 (95% CI 5.3 to 8.5). The RR was also increased for having just one of the two syndromes, 1.8 for IBS (95% CI 1.4 to 2.3) and 2.2 for chronic fatigue (95% CI 1.7 to 2.8).

Conclusions Infection with Giardia lamblia in a non-endemic area was associated with a high prevalence of IBS and chronic fatigue 3 years after acute illness, and the risk was significantly higher than in the control group. This shows that the potential consequences of giardiasis are more serious than previously known. Further studies are needed, especially in areas where giardiasis is endemic.