SummaryFrom 1938 to 1946, 98 cases of lung-abcess were admitted for observation in the medical clinic B (Louvain).Afirst groupof 53 patients were treated without sulphonamides or penicillin.Asecond groupof 45 cases were treated in one of the 3 following ways :a)high and prolonged doses of sulphonamides : 23 cases ;b)alternated sulphonamides and penicillin : 12 cases;c)simultaneous sulphonamides and penicillin : 8 cases.The first group gave 26,5 % of medical successes, while the second group reached 51 %. The total mortality (medical and surgical) was of 21 cases out of 53 in the first group and 11 cases out of 45 in the second.In both groups about half the patients came under observation more than 2 months after the onset of their disease, and one third of the patients showed complications on admission.The best results were obtained in the shortest time in the group treated by simultaneous administration of sulphadiazine or sulphathiazol and penicillin. In that way we obtained 7 satisfactory results out of 8.Actually we consider the following schedule of treatment as the most favourable (for normal adults and with the necessary precautions for a long treatment with high doses of sulphonamides) :1°4 days from 210.000 to 400.000 U. of penicillin + 8 gm. to 12 gm. sulphodiazine p.d. ;2°17 days (at least) 160.000 U. of penicillin + 8 gm. of sulphodiazine p.d.Cases in which the treatment is started late after the onset of the condition are to be treated longer.One should be warned against small doses of sulpbonamides or penicillin, which give no lasting result and may cause the failure of a new medical treatment.