We developed disease-specific measures of sickness at admission based on medical record data to study mortality of Medicare patients with one of five conditions (congestive heart failure, acute myocardial infarction, cerebrovascular accident, pneumonia, and hip fracture). We collected an average of 73 sickness variables per disease, but our final sickness-at-admission scales use, on average, 19 variables. These scales are publicly available, and explain 25% of the variance in 30-day postadmission mortality for patients with acute myocardial infarction, pneumonia, or cerebrovascular accident. Sickness at admission increased following the introduction of the prospective payment system (PPS). For our five diseases combined, the 30-day mortality to be expected because of sickness at admission was 1.0% higher in the 1985-1986 period than in the 1981-1982 period (16.4% vs 15.4%), and the expected 180-day mortality was 1.6% higher (30.1% vs 28.5%). Studies of the effects of PPS on mortality must take this increase in sickness at admission into account.