Writing therapy to improve physical or mental health can take many forms. The most researched model of therapeutic writing (TW) is unfacilitated, individual expressive writing (written emotional disclosure). Facilitated writing activities are less widely researched. Databases, including MEDLINE, EMBASE, PsycINFO, Linguistics and Language Behaviour Abstracts, Allied and Complementary Medicine Database and Cumulative Index to Nursing and Allied Health Literature, were searched from inception to March 2013 (updated January 2015). Four TW practitioners provided expert advice. Study procedures were conducted by one reviewer and checked by a second. Randomised controlled trials (RCTs) and non-randomised comparative studies were included. Quality was appraised using the Cochrane risk-of-bias tool. Unfacilitated and facilitated TW studies were analysed separately under International Classification of Diseases, Tenth Revision chapter headings. Meta-analyses were performed where possible using RevMan version 5.2.6 (RevMan 2012, The Cochrane Collaboration, The Nordic Cochrane Centre, Copenhagen, Denmark). Costs were estimated from a UK NHS perspective and three cost-consequence case studies were prepared. Realist synthesis followed Realist and Meta-narrative Evidence Synthesis: Evolving Standards guidelines. To review the clinical effectiveness and cost-effectiveness of TW for people with long-term conditions (LTCs) compared with no writing, or other controls, reporting any relevant clinical outcomes. To conduct a realist synthesis to understand how TW might work, and for whom. From 14,658 unique citations, 284 full-text papers were reviewed and 64 studies (59 RCTs) were included in the final effectiveness reviews. Five studies examined facilitated TW; these were extremely heterogeneous with unclear or high risk of bias but suggested that facilitated TW interventions may be beneficial in individual LTCs. Unfacilitated expressive writing was examined in 59 studies of variable or unreported quality. Overall, there was very little or no evidence of any benefit reported in the following conditions (number of studies): human immunodeficiency virus (six); breast cancer (eight); gynaecological and genitourinary cancers (five); mental health (five); asthma (four); psoriasis (three); and chronic pain (four). In inflammatory arthropathies (six) there was a reduction in disease severity [n = 191, standardised mean difference (SMD) -0.61, 95% confidence interval (CI) -0.96 to -0.26] in the short term on meta-analysis of four studies. For all other LTCs there were either no data, or sparse data with no or inconsistent, evidence of benefit. Meta-analyses conducted across all of the LTCs provided no evidence that unfacilitated emotional writing had any effect on depression at short- (n = 1563, SMD -0.06, 95% CI -0.29 to 0.17, substantial heterogeneity) or long-term (n = 778, SMD -0.04 95% CI -0.18 to 0.10, little heterogeneity) follow-up, or on anxiety, physiological or biomarker-based outcomes. One study reported costs, no studies reported cost-effectiveness and 12 studies reported resource use; and meta-analysis suggested reduced medication use but no impact on health centre visits. Estimated costs of intervention were low, but there was insufficient evidence to judge cost-effectiveness. Realist synthesis findings suggested that facilitated TW is a complex intervention and group interaction contributes to the perception of benefit. It was unclear from the available data who might benefit most from facilitated TW. Difficulties with developing realist synthesis programme theory meant that mechanisms operating during TW remain obscure. Overall, there is little evidence to support the therapeutic effectiveness or cost-effectiveness of unfacilitated expressive writing interventions in people with LTCs. Further research focused on facilitated TW in people with LTCs could be informative. This study is registered as PROSPERO CRD42012003343. The National Institute for Health Research Health Technology Assessment programme.