Toxoplasma encephalitis is the most common opportunistic infection of the central nervous system in patients with AIDS. The treatment of choice is a combination of sulfadiazine and pyrimethamine. We present here four patients with AIDS treated for toxoplasmic encephalitis who developed sulfadiazine-induced crystalluria. This complication was rapidly reversible with rehydration and urine alkalinization. Patients with AIDS treated with high doses of sulfadiazine should be adequately hydrated, and their urinary pH maintained above 7.5 to prevent sulfadiazine-induced crystalluria.