This multicentre, double-blind, randomized study compared the pharmacokinetics of itraconazole given at 200 mg once daily for 3 months and intermittently at 200 mg twice daily for 1 week per month followed by a 3-week drug-free period for 3 months in the treatment of onychomycosis. Patients were followed for 9 months after treatment. Itraconazole and hydroxy-itraconazole plasma concentrations and itraconazole nail tip concentrations were determined at regular intervals. With intermittent therapy (n = 64), increases of consistent magnitude were seen in the mean itraconazole and hydroxy-itraconazole plasma concentrations at the end of each 1-week treatment phase; values returned towards baseline during each subsequent 3-week drug-free period. The mean concentration of itraconazole in fingernail tips increased steadily from week 4, reached a maximum value at week 24 (213 ng/g), declined sharply between weeks 24 and 36 and returned to baseline by week 48; the mean concentration profile was similar for toenail tips (maximum value 305 ng/g at week 24) but decreased at a slower rate. With continuous therapy (n = 65), steady-state mean plasma concentrations of itraconazole and hydroxy-itraconazole were obtained within 4-5 weeks of the start of treatment and remained reasonably constant between weeks 4 and 12. The mean concentration of itraconazole in fingernail tips reached a maximum value at week 12 (524 ng/g) and returned towards baseline by week 48; in contrast, the maximum mean concentration of itraconazole in toenail tips was 698 ng/g at week 36 and did not return to baseline by week 48. No clear relationship was observed between response to treatment and concentration of itraconazole or hydroxy-itraconazole in plasma or itraconazole in nails, suggesting that concentrations exceeded therapeutic levels. In conclusion, intermittent therapy resulted in higher maximum itraconazole plasma concentrations but lower total drug exposure, and hence lower itraconazole nail concentrations, than continuous therapy. However, the intermittent schedule was not associated with a lower cure rate, which indicates that itraconazole nail concentrations remained within the therapeutic range.