We describe a randomized, double-blind, placebo-controlled, crossover trial with spiramycin in a single patient with acquired immune deficiency syndrome (AIDS) and a severe secretory diarrhea caused by cryptosporidium. Spiramycin, a potentially harmful antibiotic, had no clinical or microbiological effect in this patient. The application of the single patient (N of 1) trial to common clinical problems is a simple way to analyze the value of different therapeutic approaches. The time-consuming, expensive, multi-patient trial with ultimate extrapolation to the individual patient can be avoided. Single-patient trials can influence management and improve patient care and have potentially wide use in patients with gastrointestinal disease.