A sequencial strategy for the diagnosis of hyperthyroidism has been prospectively appraised on 410 patients using a pocket calculator-aided diagnostic system. It was found that for 64% of the patients final diagnosis could be established from nine clinical signs, ankle jerk time and free thyroxin index. For the 36% of doubtful subjects, T3 determination permitted the reduction of uncertainty to 9%. No misdiagnosis was observed. By comparing this strategy with the physician's usual diagnosis process, in which all clinical signs and several thyroid function tests were used, it appeared that the number of tests was reduced by 65% for T3 requests and by 70% for other tests (99mTc uptake and TRH). The cost-saving was estimated to be about 28%. The interest of this calculator-aided decision model resides in the possibility for the general practitioner to refer only doubtful and hyperthyroid subjects to a thyroid unit.