Cystic fibrosis patients (children and young adults) with Pseudomonas spp. chest infections were treated with meropenem or ceftazidime. This study was the first to investigate the use of meropenem in cystic fibrosis. Meropenem was well tolerated with only transient elevations of serum transaminases. No patient experienced nausea and vomiting, even when meropenem was administered as a bolus injection. This allowed home therapy to be used. Meropenem appeared to be at least as active as ceftazidime even at the low doses used. Patients showed a greater improvement in respiratory function on meropenem than ceftazidime. Only one patient (out of 60 courses) failed to respond to meropenem (98% success rate) compared with two failures out of 21 episodes with ceftazidime (90% success rate). There was little emergence of resistance to meropenem even though some patients were treated up to eight times over a 2 year period.