Malaria and leptospirosis are both common in the tropics. Simultaneous infections are possible, although not previously reported. We report two cases of malaria from an area of Thailand on the Thailand-Myanmar border with compelling serologic evidence of simultaneous acute leptospirosis. One was a case of infection with Plasmodium falciparum with acute and convalescent microscopic agglutination test (MAT) titers for Leptospira serovar icterohaemorrhagiae of 1:200 and 1:1,600, respectively. The other was a case of infection with P. vivax that seroconverted to a titer of 1:3,200 for Leptospira serovar bataviae. Additionally, there were five probable cases of leptospirosis with patent malaria parasitemia (three P. falciparum and two P. vivax) detected. Management of dual infections is complicated by their similar clinical presentations, and because the confirmatory diagnosis of malaria is readily available as opposed to that of leptospirosis. Treatment focusing on malaria mono-infections instead of dual infections could result in a delay of specific therapy for leptospirosis and possible consequences of serious complications.