Piperazine-based 'party pills' containing 1-benzylpiperazine (BZP) and 1-(m-trifluorophenylmethylphenyl)piperazine (TFMPP) have become increasingly popular in New Zealand and many other countries. The aim of this study was to collect data on the population prevalence and related harm from legal party pill use in New Zealand. A national household sample of 2010 people aged 13-45 years old was collected using a computer-assisted telephone interviewing (CATI) facility. Twenty per cent of the sample had tried legal party pills and 15% had used them in the previous year. Approximately 40% of males aged 18-24 years old had used legal party pills in the past year. While most users reported fairly minor problems from use, such as insomnia (50% of last year users), some users reported potentially more serious physical problems, such as 'vomiting' (12%), 'inability to urinate' (10%), 'chest pains' (4%) and 'seizures' (0.8%). Users also reported a range of psychological problems from use such as visual hallucinations (9%), paranoia (8%) and depression (8%). Two per cent of last year users were classified as dependent on legal party pills using a short dependency scale. The extent of harms and incidences of more serious problems, suggest that stricter regulation of the sale and use of legal party pills in New Zealand may be appropriate.