BACKGROUND The aim of this study was to compare clinical manifestations, laboratory data, morbidity and mortality between adults and children with visceral leishmaniasis, with a focus on kidney function. METHODS This was a retrospective cohort study with 432 patients with visceral leishmaniasis diagnosed at 1 center in the northeast of Brazil. Patients were divided into 2 groups according to age (>21 years and ≤ 21 years old). RESULTS The time between onset of symptoms and beginning of treatment was longer in adults (89.5 versus 48.5 days, P < 0.001); signs and symptoms were similar in both groups. Failure of treatment with glucantime was more common in adults (17.6% versus 8.8%, P = 0.008). Acute kidney injury was observed in 160 patients (37.0%), and it was more severe in adults. Risk factors for acute kidney injury in adults were hypokalemia, leukopenia, chills and amphotericin B use. In children, secondary infections were found to increase the risk for acute kidney injury. Overall mortality was 8.8%, and it was significantly higher in adults (12.6% versus 4.1%, P = 0.002). CONCLUSIONS The adult population had more severe laboratory abnormalities and a worse prognosis, possibly due to delay in diagnosis. Acute kidney injury is prevalent in both groups, and it is usually more severe in adults.