1. All members of a Spanish family (father, mother and six children) developed chloracne. 2. The causative agent was found to be the family's stock of olive oil, which had become contaminated with polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), pentachlorophenol, and hexachlorobenzene. 3. The more highly chlorinated PCDDs, in particular octachlorodibenzo-p-dioxin, were the predominant congeners in the oil. 4. Three members of the family exhibited either an overt or a sub-clinical disturbance of kidney function. The father also had a chronic respiratory problem. These changes could not be unequivocally attributed to the PCDDs. 5. Experimental toxicity of the oil was limited to the development of an hepatic porphyria in mice. 6. A serum sample, taken 5 years after consumption of the oil ceased, contained high levels of the PCDDs and PCDFs. Extrapolation back to ingested dose was used to validate dosage estimates. 7. The use of toxicity equivalence factors (TEFs) provided estimates of cumulative dosage to produce chloracne as 0.13-0.31 micrograms 2378-TCDD kg-1 (using EPA TEFs) or 6.7-16 micrograms 2378-TCDD kg-1 (using Nordic/NATO TEFs). 8. This is the first incident in which human toxicity is related primarily to ingestion of PCDDs and for which estimates of dosage can be made.