The relationship between the onset of adverse events to cotrimoxazole in HIV-infected patients and the subsequent development of toxoplasmosis, other AIDS-defining events and survival was studied in 592 French patients who first received cotrimoxazole during the Delta trial. Low CD4+ cell count at cotrimoxazole introduction was the only factor associated with the onset of adverse reactions. The occurrence of toxoplasmosis and first AIDS-defining events were significantly and independently linked to a low CD4+ cell count at cotrimoxazole introduction (p < 0.0001) and to previous cotrimoxazole withdrawal for adverse events (p = 0.004 and p < 0.0001, respectively), but not to previous cotrimoxazole withdrawal for reasons other than adverse events, as compared to patients who did not discontinue taking cotrimoxazole during this survey. The survival rate was significantly shorter among both patients who stopped taking cotrimoxazole for adverse events and for other reasons (p = 0.03 and p = 0.0001, respectively), as compared to patients who continued to take cotrimoxazole.