Introduction: Nosocomial infections remain a major cause of mortality and morbidity. The problem is most apparent in intensive care units (ICUs). Most ICU patients are compromised and vulnerable as a result of disease or severe trauma. One in ten people admitted to hospital is given an antibiotic for infection. The risk of acquiring a nosocomial infection in a European ICU is approximately 20%. It is vitally important that ways are found to prevent transmission between patients and personnel, and that local hygiene routines and antibiotic policies are developed. This thesis is a holistic work focused particularly on antimicrobial antibiotic resistance, antibiotic consumption and to some extent on hygiene in Swedish ICUs. Aims: The general aim of this thesis was to investigate bacterial resistance and antibiotic consumption in Swedish ICUs and to try to correlate ICU demographic data with antibiotic consumption and antibiotic resistance. Additional aims were to investigate on which clinical indications antibacterial drugs are prescribed in the ICU, and to investigate the emergence of resistance and transmission of Pseudomonas aeruginosa in the ICU using cluster analysis based on antibiograms and genotype data obtained by AFLP. Material and methods: In paper 1-3, antibiotic consumption data together with bacterial antibiotic resistance data and specific ICU-demographic data were collected from an increasing number of ICUs over the years 1997-2001. Data from ICUs covering up to six million out of Sweden’s nine million inhabitants were included. In paper 4, the indications for antibiotic prescribing were studied during two weeks in 2000. Paper 5 investigated Pseudomonas aeruginosa isolates in order to detect cross-transmission with genotype obtained by AFLP, and antibiogram-based cluster analysis was also performed in order to see if this could be a quicker and easier substitute for AFLP. Results: This thesis has produced three important findings. Firstly, antibiotic consumption in participating ICUs was relatively high during the study period, and every patient received on average more than one antimicrobial drug per day (I-IV). Secondly, levels of antimicrobial drug resistance seen in S. aureus, E. coli and Klebsiella spp remained low when data were pooled from all ICUs throughout the study period, despite relatively high antibiotic consumption (I-V). Thirdly, the prevalence of antibiotic resistance in CoNS and E. faecium, cefotaxime resistance in Enterobacter, and ciprofloxacin and imipenem resistance in P. aeruginosa was high enough to cause concern. Conclusion: For the period studied, multidrug resistance in Swedish ICUs was not a major problem. Signs of cross-transmission with non-multiresistant bacteria were observed, indicating a hygiene problem and identifying simple improvements that could be made in patient care guidelines and barrier precautions. A need for better follow up of prescribed antibiotics was evident. With further surveillance studies and monitoring of antibiotics and bacterial resistance patterns in the local setting as well as on a national and international level, some of the strategic goals in the prevention and control of the emergence of antimicrobial-resistant microbes may be achievable.