To determine whether nosocomial infections due to Acinetobacter species have increased over the past 10 years and whether infections continue to have a pronounced seasonal variation, we analyzed infections reported by hospitals in the National Nosocomial Infections Surveillance System that performed adult and pediatric intensive care unit surveillance from 1987 through 1996. Overall, 3447 nosocomial acinetobacter infections were reported during 5,596, 156 patient-days. There was a yearly median of 7.2 infections (range, 5.0-10.5) per 10,000 patient-days and a downward trend in the rate of acinetobacter infections overall (P<.05) and of 2 major types of infection (P<.05): bloodstream infections (yearly median, 1.6 per 10, 000 central venous catheter-days; range, 1.3-2.9) and pneumonia (yearly median, 7.6 per 10,000 ventilator-days; range, 6.5-12.0). Throughout this period, average rates were significantly higher during July-October than during November-June for acinetobacter infections overall (8.0 vs. 5.2; P<.01) and for bloodstream infections (2.0 vs. 1.2; P<.01) and pneumonia (9.7 vs. 6.6; P<.01).