We evaluated the frequency of seroconversion for toxoplasmosis in seronegative recipients of thoracic solid organ transplants with seronegative or seropositive donors and the efficacy of chemoprophylaxis with pyrimethamine+sulfametopirazine. One hundred and sixty one patients seronegative for toxoplasmosis were followed-up at different intervals. Six patients out of 79 R-/D- and twelve out of 82 R-/D+ seroconverted after chemoprophylaxis interruption. There was no difference between matched and mismatched recipients as to the frequency of seroconversion which therefore could not be related to donor seropositivity. Seroconversions were almost asymptomatic. All positive recipients should be tested if symptoms of infection are present.