The hospitalised patients in a cardiological hospital (Lille, France) over an 18-month period were subjected to a prospective high-intensity adverse drug reaction (ADR) monitoring in order to assess the additional financial resource utilisation associated with ADRs and analyse the distribution of excess of cost according to ADR nature and therapeutic classes. Over 18 months, among the 16,916 hospitalised patients, 371 verified ADRs detected by self-report stimulated by a special unit of nurses and pharmacologists occurred in 336 patients with an overall ADR rate of 2.2%. This rate increased with age. The most common reactions were cutaneous events (24%), cardiovascular events (21%), metabolic disorders (12%), coagulation disorders (10%) and nervous system impairment (10%). The most common drug classes involved were cardiovascular agents (36%), contrast media (20%), drugs affecting blood clotting (13%) and anti-infectives (14%). Increased ADR-induced costs result especially from prolongation of length of stay and cost increase was evaluated at Euro 4150 per ADR. Among the 371 ADRs, 134 ADRs, which were significantly more severe, induced a prolongation of length of stay. Renal insufficiency and cardiovascular events were significantly over-represented in this sub-group. The most common ADR-inducing drugs associated with a prolongation of length of stay are cardiovascular agents and drugs affecting blood clotting. In contrast, cutaneous ADRs were significantly over-represented in the group of ADRs without prolongation of length of stay. The severity and substantial costs of ADRs in hospital justify investments to prevent these events. Nevertheless, only a portion of ADRs induces cost increases, suggesting that prevention efforts should focus on this limited category of ADRs.