Background Renal impairment often goes unrecognised until the most advanced stages. Patients with renal impairment use a variety of drugs and the pharmacokinetics and drug effects are prone to change. Objective Describe drug related problems in a population of patients with renal impairment, investigate possible risk factors, and investigate whether the clinical pharmacist can propose relevant measures to solve the problems. Setting The internal medicine department at Innlandet Hospital Trust Gjøvik, Norway. Method A prospective study enrolling patients (≥18 years) with moderate and severe renal impairment, i.e. with glomerular filtration rate of 30–59 and 15–29 ml/min/1.73 m², respectively, and using at least one drug. The clinical pharmacist reviewed the patients’ drug regimen with focus on drug related problems, related to renal function. Problems identified were discussed with the multidisciplinary team, or directly with the physician. Main outcome measure The number of drug related problems. Results The results are based on data from 79 patients, 21 with severe and 58 with moderate renal impairment. Most patients, 92.4 %, used 5 or more drugs; in average 10.2 (range 2–27). In total, 88 drug related problems were identified in 49 patients, i.e. in 62 %. The most frequently occurring problems were incorrect dose (45.5 %) and inappropriate drug (41.0 %). There was a significant correlation between both the degree of renal impairment and the number of drugs, and the number of drug related problems. The acceptance rate of the clinical pharmacist’s interventions was 95.7 %. The drugs most frequently associated with drug related problems were metformin, benzylpenicillin and furosemide. Conclusion Drug related problems often occur in patients with renal impairment. Incorrect drug dose and inappropriate drug choice according to the patients’ renal function were the most common problems. Patients with the most complex drug treatment, i.e. with increasing degree of renal impairment and increasing number of drugs, are at greater risk of drug related problems. The high acceptance rate for the pharmacist’s interventions supports the inclusion of a clinical pharmacist in the multidisciplinary treatment team to increase awareness and optimisation of the drug treatment in this patient group.