There is controversy about whether hospitalized poor patients use more resources and whether hospitals that provide care for the poor therefore merit supplementary payment under per-case prospective payment systems. We previously reported that patients of low socioeconomic status with connective-tissue disease had longer hospital stays and higher costs than patients of higher socioeconomic status at a single hospital. To examine the generalizability of this phenomenon, we interviewed 16,908 (83 percent) of 20,278 consecutive adult patients (excluding obstetrical and psychiatric patients) admitted in 1987 to five Massachusetts hospitals, to obtain information on three direct measures of socioeconomic status (income, occupation, and education). We divided each measure into three strata. Thus, there were 15 comparisons--three measures of socioeconomic status applied to each of five hospitals. After excluding outliers and adjusting for diagnosis-related group (DRG), we found that the patients of the lowest socioeconomic status had hospital stays 3 to 30 percent longer than those of patients of higher status, the differences varying with the hospital and the indicator of socioeconomic status (P less than or equal to 0.05 for 11 of the 15 comparisons). Hospital charges were 1 to 18 percent higher for the patients of lowest socioeconomic status than for those of higher status (P less than or equal to 0.05 for 9 of 15 comparisons). When we adjusted for age, severity of illness, and DRG, the patients of lowest socioeconomic status had longer stays than those of higher status in 14 of 15 comparisons (P less than 0.05 for 7 of the 15) and higher charges in 13 of 15 comparisons (P less than 0.05 for 6 of the 15). The differences between patients of high and low status ranged up to 21 percent for length of stay and 13 percent for charges. Our findings suggest that hospitalized patients of lower socioeconomic status have longer stays and probably require more resources. Supplementary payments to hospitals for the treatment of poor patients merit further consideration.