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Up to 75 percent of patients with AIDS will develop the infection Pneumocystis carinii pneumonia, or PCP. Treatment of PCP consists of either pentamidine or trimethoprim-sulfamethoxazole (TMP-SMX). Studies have been done to find a method to prevent PCP, in addition to treating it. In other patients with compromised immune systems, such as those with cancer or those who have received transplants, TMP-SMX given daily or several times per week has proven to be beneficial to prevent the disease. Few studies have been done thus far to show if TMP-SMX is effective in preventing PCP in AIDS patients. TMP-SMX was given orally every other day to two groups of AIDS patients. One group received the drug as primary prophylaxis, having never had PCP. The second group was given the drug as secondary prophylaxis, in that they had already had PCP and the drug was administered in the hopes of preventing recurrence. Of the patients given TMP-SMX as primary prophylaxis, none developed PCP during the time of the study, while three (31 percent) had the drug discontinued due to adverse reactions. The patients given TMP-SMX had a much higher incidence of adverse reactions, as 52 percent of them had to discontinue the drug before the study period was over. Of those who were able to tolerate the drug, only one of 27 patients developed PCP. The greater incidence of adverse reactions in the group receiving secondary prophylaxis might be attributable to the fact that most of them had been treated previously with TMP-SMX for their first episodes of PCP. Given that the existing drug recommended for PCP prophylaxis is pentamidine in an aerosol form, which costs over $100 per month, and that TMP-SMX costs approximately $1.20 per month, TMP-SMX should be the drug of choice in those who can tolerate it without adverse effects. (Consumer Summary produced by Reliance Medical Information, Inc.)