Background. Pharmaceutical representative visits are believed to have substantial impact, but the effects on prescribing patterns have not been systematically evaluated. Objective. This study investigates how pharmaceutical sales representative visits influenced physicians' company-specific drug preferences and prevalence of steroid prescribing. Methods. Observational cohort study in Funen County, Denmark, including 165 general practices visited 832 times by pharmaceutical representatives and 54 080 patients treated with asthma drugs. Visits were conducted from 2001 to 2003. Our main outcome measures were (i) company-specific drug preferences measured as the proportion of dispensings of the promoted drug among all dispensings of fixed combinations of inhaled corticosteroid and long-acting β₂-agonists and (ii) the proportion of patients receiving repeated β₂-agonist dispensings who were treated with inhaled steroids. Results. The first visit had a statistically significant effect on the GPs' drug preference in favour of the marketed drug [odds ratio (OR), 2.39; 95% confidence interval (CI), 1.72-3.32]. The effect on drug preference increased further after the second visit (OR, 1.51; 95% CI, 1.19-1.93), while there was no significant change after the third visit (OR, 1.06; 95% CI, 0.94-1.20). Pharmaceutical sales representative visits did not influence the overall treatment pattern with inhaled steroids (OR, 1.01; 95% CI, 0.97-1.06). Conclusions. Pharmaceutical sales representative visits markedly increased the market share of the promoted drug, but only the two first visits had significant impact. Visits had no significant impact on GPs' overall prescribing of inhaled steroids.