Six Leishmania major and seven L. tropica parasites were isolated and identified from participants in Operation Desert Shield/Storm. A complete enzyme analysis (21 enzymes) revealed that there was enzyme polymorphism among the isolates of each species group. Any one Desert Storm L. major isolate could differ from any other for 1-3 enzymes, and any L. tropica isolate could differ from any one other for up to eight enzymes. Enzyme polymorphism data from other L. major and L. tropica isolates from Africa and the Middle East region were obtained and combined with the Desert Storm data to produce population enzyme polymorphism estimates. Results from these population data indicated that L. major parasites could be expected to differ from each other for as many as eight enzymes and still be L. major, and similarly, L. tropica isolates could differ for as many as 14 enzymes. These expected isolate variation extremes have not been observed among the isolates studied. All L. major and most L. tropica isolates were from patients who, as expected, presented with cutaneous disease, but the Desert Storm and two Kenyan patients infected with L. tropica presented with a viscerotropic disease, the symptoms of which are unlike those of classic visceral leishmaniasis. Such unrecognized presentation for these L. tropica-infected patients indicates that both parasite and patient can play critical roles in disease manifestations. The Desert Storm isolates are, as indicated, either L. major or L. tropica.