We read with interest the article by Bell and colleagues (Clin Med June 2009 pp 231–5). They describe aciclovir therapy as essentially safe, highlighting the potential risk of crystal nephropathy. This potentially life-threatening complication is well recognised in nephrology, but not widely publicised, as it is often only evident in the presence of renal impairment. Recognition has implications for all physicians given the prevalence of chronic kidney disease and acute kidney injury. Such concerns might explain five patients not receiving full dose aciclovir in their study.