Thallium poisoning seldom occurs in Spain. This article reports 5 cases of thallium poisoning, of which 4 of the patients belonged to the same family. The cases occurred in or near Granada between 1985 and 1987. The symptoms were initially gastrointestinal (vomiting, abdominal pain, gastrointestinal haemorrhage, etc.) in the case of the family poisoning, and a sensitive-motor polyneuritis in the fifth case. The diagnosis was established by analysis carried out in the authors' laboratory; urinary thallium concentrations were determined by atomic absorption spectrophotometry with graphite furnace and monovalent hollow cathode lamp. Each case was followed up to confirm the efficacy of the treatment; recovery from the poisoning was complete in all cases after 3 to 9 weeks.