Two cases of myocarditis and congestive heart failure in immunocompetent patients with seroconversion for toxoplasmosis are reported. Serological tests showed that in the first case the cardiac manifestations occurred at the time of seroconversion (low IgG, raised IgM and IgA), whereas in the second case they occurred several months after the initial infection when IgM was decreasing, IgG levels were very high (>1000 IU/ml) and IgG had high affinity for the antigen. The pathophysiological mechanisms underlying cardiac involvement in acute or chronic toxoplasmosis are discussed.