OBJECTIVES: To review the characteristics and performance of research ethics committees in Spain in the evaluation of multicentre clinical trial drug protocols. DESIGN: A prospective study of 100 applications. SETTING: Forty-one committees reviewing clinical trial protocols, involving 50 hospitals in 25 cities. MAIN MEASURES: Protocol-related features, characteristics of research ethics committees and evaluation dynamics. RESULTS: The 100 applications involved 15 protocols (of which 12 were multinational) with 12 drugs. Committees met monthly (except one). They had a mean number of 12 members, requested a mean of six complete dossiers and nine additional copies of the protocol with a mean deadline of 14 days before the meeting. All applications were approved except three (two of the three were open-label long-term safety trials rejected by the same committee), which were approved by the other committees involved. The mean time from submission to approval was 64 days. The mean time from submission to arrival of the approval document at our offices was 85 days. Twenty-five committees raised queries for 38 of the 97 finally approved applications. Impact of evaluation fee, number of members, queries raised and experience of committees on timings were not statistically significant. CONCLUSION: Obtaining ethical approval is time-consuming. There is much diversity in the research ethics committees' performance. A remarkable delay (> 20 days) exists between the decision and the arrival of the written approval, suggesting administrative or organisational problems.