Supervision of lithium treatment among three groups of patients treated in different settings – at a lithium clinic, as hospital out-patients, and by a GP – was studied. The lithium clinic maintained lower levels and checked these more frequently. Elevated lithium levels were most frequent in the GP-supervised group, which also included the 13 patients with the most impaired glomerular filtration. In hospital settings GFR-impaired patients received a lower daily dose of lithium; in general practice, affected patients had a higher mean serum lithium level and were more frequently prescribed lithium once daily. We conclude that patients with glomerular impairment who require lithium should be supervised by a specialist, or at least prescribed lithium in divided daily doses.