To evaluate the quality of pharmaceutical prescription by means of various indicators. Cross-sectional, retrospective study. Non-hospital prescription in all the INSALUD provinces, except Ceuta and Melilla, for a year (1997). Different quality indicators were analysed for the following groups of medicines: oral diabetes drugs, anti-platelet aggregation drugs, anti-hypertension drugs, non-steroidal anti-inflammatories, tranquillisers and hypnotic drugs, antibiotics, anti-asthmatic drugs and the omeprazol efficiency indicator. Consumption was expressed in defined daily doses (DDD) and in DDD per 1000 persons and per day (DID). A prescription quality indicator was indicated on the basis of weighing up the indicators. The sulphonylurea drugs varied greatly (15-56%) and deviated negatively a lot from the standard value. AAS and ticlopidine made up 74% of the anti-platelet aggregation drugs. Consumption of calcium antagonists was three times greater than of beta-blockers. Captopril and enalapril were 65% of the ACE inhibitors. Ibuprofen, naproxen and diclofenac made up between 34 and 50% of the NSAIDs. The DID of hypnotics was 44 against a standard figure of 24. Use of first-level antibiotics was 77%. Inhaled adrenergic drugs came to 80% of total anti-asthmatic drugs. The cost of omeprazol treatment per day was 267 pesetas/DID, which was inefficient in comparison with the standard of 190. All provinces had a prescription quality indicator above six out of ten. The ACE inhibitors, anti-asthmatic drugs and antibiotics were close to the standard. The use of sulphonylurea drugs, beta-blockers, NSAIDs and hypnotics was a long way from standard. The prescription level was acceptable in the different provinces.