Urine lipoperoxides were measured, as the malondialdehyde-thiobarbituric acid adduct, by "high performance" liquid chromatography in men who tested positive for any of the common "drugs of abuse." Urine malondialdehyde concentrations were within the reference range in those urines containing barbiturates and amphetamines. On the other hand, significant urine malondialdehyde elevations were associated with those testing positive for marijuana (p less than 0.005), cocaine, opiates, benzodiazepines, and in those urines containing multiple drugs (p less than 0.001). The mechanism of tissue damage by these drugs is not necessarily by lipid peroxidation, since their primary toxic effects might result in cellular injury by an alternative mechanism; the observed increased lipid peroxidation could be a secondary phenomenon following tissue injury.