Twenty-four chronic schizophrenic patients were treated successfully with the addition of acetazolamide and thiamine (A + T) to their unchanged existing therapies in a double-blind, placebo-controlled crossover study. Therapeutic effects were measured by the Scale for the Assessment of Positive Symptoms and the Scale for the Assessment of Negative Symptoms. Overall, 50% of the patients showed improvement on all assessment scales. No untoward effects occurred in these patients or in patients in previous studies who have been treated continuously with A + T therapy for as long as 3 years.