It is to be expected that in the near future the success or failure of antihypertensive substances will be mainly determined by their spectrum of side effects so long as their antihypertensive efficacy is comparable. This double-blind study with crossover of enalapril and atenolol in 58 mild to moderate hypertensive patients is, as far as we know, the first clinical trial evaluating next to the antihypertensive efficacy also compliance and side effects. On the grounds of excellent compliance results (ca. 95%) blood pressure values as well as the incidence of side effects can be assumed to be accurate. Our results show that the antihypertensive efficacy of enalapril is comparable to that of atenolol. The number of patients with side effects did not differ significantly between enalapril and atenolol.