An electronic monitoring device was used to assess patient compliance during clinical trials of two new aerosolized nonbronchodilator, antiasthma drugs. Compliance was poor, but similar, in both trials. Patients took the study drug as instructed on a mean of 37.3% days (range 10% to 77%) in one trial and 37.4% days (range 0% to 63%) in the other. Both underuse and overuse were observed. Underuse was seen on a mean of 38.9% days (range 9% to 81%) and 46.0% (range 15% to 80%) respectively; overuse on a mean of 23.5% days (range 6% to 54%) and 16.6% days (range 0% to 41%). In some patients ten or more activations of the device were recorded at the same time. In six patients such multiple simultaneous activations were recorded on two or more separate days. These patients were among the most noncompliant in the study. Multiple simultaneous activations frequently followed underuse days or preceded followup visits, suggesting the possibility of duplicity. Overall compliance was such that valid conclusions about efficacy of the drugs could only have been drawn in six of 34 patients.