Eligible patients who consent to take part inCiproPAL and who are already enrolled inALLTogether1 will be randomised (1:1) to theinterventional arm or control arm of the trial.

Patients randomised to the interventional arm willreceive prophylactic ciprofloxacin (10mg/kg twice aday, enteral or intravenously) from randomisation tothe end of induction. Specifically, prophylacticciprofloxacin should commence as soon as possibleafter randomisation (within the first 5-8 days ofinduction therapy) and continue until neutrophilrecovery sufficient for the commencement ofconsolidation (ANC >0.5x109/L).

Patients randomised to the control arm will receiveno additional treatment during induction. Suchpatients should receive the standard of care, as perlocal policy.

Any febrile episodes should be treated according tolocal guidelines. For patients in the interventionalarm (receiving prophylactic ciprofloxacin),ciprofloxacin prophylaxis should be stopped for theduration of any treatment with broad spectrumantibiotics, empirically or for a defined infection, andre-started afterwards. If antibiotic coverage isrationalised to treat a specific infection, for exampleteicoplanin for a Coagulase negativeStaphylococcus central line infection, thenciprofloxacin should be restarted. If there are anyuncertainties regarding restarting ciprofloxacinplease contact UCL CTC who will enquire with thetrial management group (TMG).

Inclusion:

• Paediatric patients (1-17 years inclusive) withde-novo Acute Lymphoblastic Leukaemiatreated on ALLTogether1 in the UK in the first 5-8 days of therapy.

• Written informed consent.

Exclusion:

• Non-participants of the ALLTogether1 trial.

• Patients with Down syndrome.

• Patients with chronic active arthritis.

• Patients with any other contraindication tofluoroquinolones.