Morphine and codeine are frequently identified in blood samples from impaired drivers. But whether these opiates reflect the use of prescription analgesics or abuse of the illicit drug heroin (diacetyl morphine) is not always obvious. Opiates, either alone or together with other drugs, were determined in 2573 blood specimens from impaired drivers by sensitive and specific methods of analysis. The specific metabolite of heroin 6-acetyl morphine (6-AM) was quantifiable in only 52 cases (2%) at mean, median, and highest concentrations of 0.015, 0.010, and 0.10 mg/L, respectively. The mean, median, and highest concentrations of morphine were 0.046, 0.03, and 1.13 mg/L, respectively (N = 2029). The corresponding concentrations of codeine (N = 1391) were 0.047, 0.01, and 2.40 mg/L. Ethyl morphine was identified in 63 cases at a mean concentration of 0.055 mg/L (median 0.03 mg/L). When 6-AM was present in urine (N = 324), the mean morphine/codeine ratio in blood was 7.5 (median 6.7), and this important ratio was less than unity in only two cases. This study finds compelling evidence that ∼90% of apprehended drivers in Sweden with morphine and codeine in their blood had used heroin.