"Recent fatal human hantavirus pulmonary syndrome (HPS) cases, one each in Colorado and New Mexico, plus a non-fatal HPS case in Arizona and a non-fatal (at this time) HPS case in New Mexico has people in this part of the world (southwestern USA) justifiably nervous. Ecologic conditions vary from local to location, of course, but in general, recent observations have been of increases in rodent populations and increased prevalence of antibody to Sin Nombre virus, likely a result of two wet, relatively warm El Nino/Southern Oscillation winters. The next month should allow determinations as to whether conditions are suitable for a repeat of the 1993 situation, when HPS and Sin Nombre virus were first recognized. Unfortunately, the only way to test the predictive model is to experience another outbreak. After that, prediction should be easier." Charles H. Calisher <calisher@usa.healthnet.org> VIA ProMED-mail <promed@usa.healthnet.org>.