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Little is known about the antibiotic susceptibility and clinical significance of non-type b capsulated Haemophilus influenzae. We studied the antibiotic resistance patterns, plasmid carriage and clinical features of H. influenzae type f infections in Spain during 1996–2002.Forty-nine H. influenzae type f recovered from Spanish hospitals were analysed at a central laboratory where full microbiological and molecular epidemiological studies were carried out. Antimicrobial susceptibility testing was performed in accordance with NCCLS guidelines.Twelve strains (24.5%) were resistant to ampicillin and 22 (44.9%) to co-trimoxazole. Decreased susceptibility to clarithromycin, tetracycline, chloramphenicol and rifampicin was found in 16.3%, 12.2%, 14.3% and 2% of strains, respectively. Multidrug resistance was present in nine (18.4%) of the 49 isolates. The most prevalent resistance phenotype was ampicillin/tetracycline/co-trimoxazole/chloramphenicol, which was detected in five isolates. All six strains that were simultaneously resistant to ampicillin, tetracycline and chloramphenicol had conjugative plasmids. The main clinical diagnoses were pneumonia (32.6%), sepsis (18.4%) and meningitis (16.3%). Thirty-two patients (65.3%) had previous underlying predisposing conditions, principally respiratory diseases (20.4%). Twenty-one patients (42.8%) had impaired immunity. Thirty-seven (75.5%) patients were >14 years old, 12 (24.5%) were ≤14 years, and seven were ≤5 years. Most isolates were clonally related.A high prevalence of antibiotic resistance, including multiresistance, was detected in Spanish H. influenzae type f isolates. Carriage of large conjugative plasmids was strongly associated with antibiotic resistance. H. influenzae type f is mainly an opportunistic pathogen, although it may cause primary severe infections, such as meningitis in children.