OBJECTIVE To evaluate the epidemiology of Giardia lamblia infection, investigate factors which might be associated with clinical manifestations and recurrence, and examine the role of copathogens in disease course.

METHODS Prospective 4‐year cohort study of children born in an urban slum in north‐eastern Brazil.

RESULTS Of 157 children followed for ≥ 3 months, 43 (27.4%) were infected with Giardia. The organism was identified in 8.8% of all stool specimens, and although found with similar frequency in non‐diarrhoeal (7.4%) and diarrhoeal stools (9.7%), was more common in children with persistent (20.6%) than acute diarrhoea (7.6%, P=0.002). Recurrent or relapsing infections were common (46%). Children with symptomatic infections had significantly lower weight‐for‐age and height‐for‐age than asymptomatic children. Copathogens were not associated with disease course.

CONCLUSION With its protean clinical manifestations, Giardia may be associated with substantial morbidity amongst children in Brazil.