Babesiosis is a malaria-like illness caused by the intraerythrocytic parasite Babesia microti and transmitted by the same tick that transmits the Lyme disease spirochete. Both pathogens may be cotransmitted and are endemic in southern New England and New York, the northern Midwest and the far West. The epidemiology and clinical presentation of babesiosis and babesiosis/Lyme disease coinfection are well described in adults but not in children. Accordingly, we reviewed data from longitudinal, clinic-based serosurvey and case-finding studies of tick-borne illness in Connecticut, Massachusets and Rhode Island from 1990 through 1996. A total of 147 episodes of babesial infection were identified, 125 in adults and 22 in children. Approximately a third of children and adults with babesial infection reported symptoms. The most frequently reported symptoms in children were fever (25%), fatigue (19%) and headache (19%) and in adults, fatigue (20%), fever (17%) and myalgias (16%). Children and adults had a similar number of acute symptoms; the average number of symptoms were 4.0 and 4.8, respectively. Children recovered more rapidly, however, as none were ill for more than a month compared with 20% of adults. Coinfection with babesiosis and Lyme disease resulted in more prolonged illness; 17% of children and 26% of adults reported symptoms that lasted for more than a month. We conclude that the frequency of babesial infection is similar in children and adults but children have a shorter duration of illness. Both children and adults experience a more prolonged illness when babesiosis is accompanied by Lyme disease.