OBJECTIVE To check the validation of the precision in measuring arterial pressure (AP) of an automatic non-invasive machine for out-patient monitoring of arterial pressure, the ACP-2200, comparing the sequential with the simultaneous method in line with the criteria of the Association for the Advancement of Medical Instrumentation. METHODS A comparative crossover study. METHODS Primary Care. "La Orden" Health Centre, Huelva. METHODS 86 individuals, 42 men and 44 women of all ages, both hypertense and not, and without any antihypertension treatment, with SAP intervals between 99 and 160 mmHg and DAP between 59 and 105 mmHg, selected at random from the on-demand consultations. RESULTS Three AP readings were made with a manual apparatus for reference (a mercury sphygmomanometer) and three simultaneously with the ACP-2200 and the manual apparatus placed in a T on the cuff. The AP means obtained were compared and the mean calculated. The standard deviations of the paired differences between the APs obtained with the ACP-2200 and with the manual method, both in the sequential and simultaneous form, as well as Pearson's lineal correlation coefficient, were all calculated. The means of the paired differences and the standard deviations between the automatic and manual apparatuses were, for the SAP, 1.52 +/- 4 (r = 0.96) and 0.66 +/- 3.38 (r = 0.97) (sequential and simultaneous, respectively) and, for the DAP, 0.86 +/- 2.84 (r = 0.98) and 0.64 +/- 1.73 (r = 0.99) (sequential and simultaneous, respectively). All the values of p were inferior to 0.001. CONCLUSIONS The ACP-2200 complies with the validation criteria for the two methods used, with the simultaneous method being recommended for use.