Out of 69 patients with spontaneous systemic lupus erythematosus (SLE), phenotyped for polymorphic acetylation with sulphadimidine, 52 (75%) were slow acetylators. In the subgroup of SLE patients with chronic biologically false positive seroreactions for syphilis the percentage of slow acetylators was even higher, 88%. In the majority of the slow acetylators the disease had started later and had followed a milder course than in rapid acetylators. Cutaneous reactions suspected to be drug-induced were seen in 19 (17 slow acetylators) during an observation period of 3--7 years. The reactions were mostly of exanthematous or urticarial type but also fixed type of eruption was seen. Provocation tests with the suspected drug were performed in 14 patients. In 5 cases it could be demonstrated that the eruption was caused by the drug. The predominance of slow acetylators among our patients with spontaneous SLE was the same as has been observed in drug-induced SLE. This suggests a similar genetic background.