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We present multiwavelength (optical, IR, radio) observations of CI Cam, the optical counterpart to the transient X-ray source XTE J0421+560. Pre-outburst quiescent observations reveal the presence of a dusty envelope around the system. Pronounced short term variability is observed at all wavebands from U–K, but no indication of prior flaring of a similar magnitude to the 1998 April outburst is found in these data. Data obtained during the 1998 April X-ray flare reveal pronounced optical-radio flaring. The optical flux was observed to quickly return to quiescent levels, while the radio flare was of much longer duration. The optical component is likely to result from a combination of free–free/free–bound emission, emission line and thermal dust emission, caused by re-radiation of the Xray flux, while the behaviour of the multiwavelength radio data is consistent with emission from expanding ejecta emitting via the synchrotron mechanism. Post-outburst (1998 August–1999 March) U–M broadband photometric observations reveal that while the optical (UBV ) flux remains at pre-outburst quiescent levels, near IR (JHKLM ) fluxes exceed the pre-outburst fluxes by ∼0.5 magnitudes. Modelling the preand post-outburst spectral energy distribution of CI Cam reveals that the structure and/or composition of the dusty component of the circumstellar envelope appears to have changed. Due to a lack of information on the precise chemical composition of the dust within the system several explanations for this behaviour are possible, such as the producSend offprint requests to: J.S. Clark Correspondence to: ([email protected]) tion of new dust at the inner edge of the envelope, or modification of the composition of the dust due to X-ray irradiation.