Of the 523,672 hospital discharges in Washington State in 1996, a total of 117,222, or 22 percent, had substance abuse diagnoses. As expected, more hospitalizations with substance abuse occurred during the first week of the month than in the last week of the preceding month (R=103.5, 95% CI=101.7 to 105.4). For all other diagnoses, hospitalizations in the two time periods were similar (R=100.4, 95% CI=99.4 to 101.3).